Zodiac Constellation #1: Gemini

By Peter Forrester | February 6, 2019

Greetings, stargazers!

In case you’re wondering about my choice of constellations, I pointed out in the article about the Zodiac that the constellations each have the Sun in their direction for about one month; although that month is off by a little bit from the traditional astrological dates. I therefore can’t write about it in its normal month because it can’t be seen then. I will be writing about a constellation that is visible in the east in the early evening, around 6 or 7 pm in US Eastern Time.

Gemini is Latin for “twins”, and indeed, the two brightest stars in Gemini are very similar in brightness, if not in detail. Castor and Pollux were twins from Greek mythology, hunters and later sailors on Jason’s Argo. They were said to have been placed together in the sky after one of them was killed. Oddly, they are also often said to have had different fathers, with one of the fathers being the god Zeus.

Pollux is the brightest star in Gemini, with an apparent magnitude of 1.14. It is an orange giant star about 34 light-years from Earth. Castor is just behind it at 1.58, but actually consists of SIX different stars, in three binary pairs. They are 52 light-years away, and appear to be a single blue-white star.

The brightest stars in each constellation are assigned Greek letters, with the brightest one usually being designated as Alpha (α), but Gemini is an exception, with Castor being α Geminorum, or in short form α Gem (you’ll often see this short form next to the stars on sky charts). Pollux, the brightest star, gets the next letter, Beta (β Geminorum or β Gem). After proceeding through all 24 letters of the Greek alphabet, the dimmer stars will be assigned numbers.

The remainder of the constellation principally consists of two lines of stars, one extending out to the right of each of the bright stars (in the northern hemisphere). There are 85 stars in Gemini that are visible without using a telescope. Four of the stars are brighter than 3.00. Eight of the stars are known to have planets, one of them being Pollux.

In drawings of the constellation, Castor and Pollux are the heads of the twins, with the parallel line going down along towards their feet at the other end. Their feet are pointed right at the constellation Orion.

Gemini is easy to find in the sky if you can identify Orion (February is the best month to see Gemini, so I recommend braving the cold). From northern latitudes, it appears just to the east (left) and slightly above Orion. If you make a line from the right side of Orion’s belt, or the right foot, and pass through the left shoulder (from your perspective), it will lead straight to Castor and Pollux.

Gemini is important in astronomical history: the planet Uranus and the former planet, now dwarf planet Pluto, were both discovered in Gemini, in 1781 and 1930, respectively. Much earlier, Aristotle wrote about the earliest recorded occasion of a planet occulting, or passing in front of a star. The planet Jupiter passed in front of a star in Gemini; it is believed that the star was the one called 1 Geminorum (not the 25th brightest – its number comes from a catalog that was done from west to east), and that the event occurred on 5 December, 337 BC.

Next in series: Zodiac Constellation # 2: Cancer


Encyclopaedia Britannica. (2009, February 18). Castor (star). Retrieved from https://www.britannica.com/place/Castor-star.

Wikipedia. (2018, July 27). 1 Geminorum. Retrieved from https://en.wikipedia.org/wiki/1_Geminorum.

Wikipedia. (2019, January 10). Castor (star). Retrieved from https://en.wikipedia.org/wiki/Castor_(star).

Wikipedia. (2019, January 29). Gemini (constellation). Retrieved from https://en.wikipedia.org/wiki/Gemini_(constellation).

Wikipedia. (2019, January 2). Pollux (star). Retrieved from https://en.wikipedia.org/wiki/Pollux_(star).

Skies Over Milton, February Edition

By Peter Forrester | February 3, 2019

Here are your skywatching events for the month of February. Happy viewing! I hope you will take advantage of a few warm days we have coming up to get out there and look at the stars.


Friday, February 1: Moon near Venus (morning).

Saturday, February 2: Moon near Saturn (morning).


Monday, February 4: New Moon at 4:04 PM (Eastern Standard Time)

Tuesday, February 5: Moon at farthest from Earth, 4:00 PM.

Sunday, February 10: Moon near Mars at 5 PM.

Tuesday, February 12: First Quarter Moon at 5:26 PM.

Wednesday, February 13: Moon near the Pleiades star cluster, 6:00 AM.

Monday, February 18: Venus near Saturn, 8:00 AM.

Tuesday, February 19: Moon at closest to Earth, 3:51 AM. Also Full Moon at 10:54 AM.

Tuesday, February 26: Last Quarter Moon at 6:28 AM. Also Mercury furthest from the Sun in the sky, 8:00 PM. Note that this planet can only be seen with the naked eye for a brief period during morning or evening twilight, as it never gets far from the Sun. On this occasion it is only 18 degrees away from the Sun.

Wednesday, February 27: Moon near Jupiter, 11:00 AM.

Not all of these events will be visible in Milton, depending on when the daylight occurs. For more information, and additional events, see the Skymaps link below.

 Previous in series: Skies over Milton, January Edition

| Next in series: Skies Over Milton, March Edition


Thalassoudis, Kym. (2000-18). Skymaps. Retrieved February 3, 2019 from skymaps.com.

Wikipedia. (2019, February 1). Mercury. Retrieved from https://en.wikipedia.org/wiki/Mercury_(planet).

Observing the Moon, Part 1: Appearance and Phases

By Peter Forrester | January 27, 2019

I skipped over the Sun and the Moon when I wrote about the planet Venus. They are the only two natural objects in the sky brighter than Venus. I will deal with the Sun when we are getting close to a solar eclipse, as that is the only time it is safe to look at it (unless you saved eclipse glasses from the last one, and make sure there are no holes in them. Do not try to use any other type of glasses).

The Moon is our nearest neighbor in space (about 238,000 miles from Earth, on average), and it is the only place outside of Earth orbit where humans have visited (only during six U.S. Apollo missions between 1969 and 1972). It is also the only object, other than the Sun, which is close enough to appear as more than a point of light when seen by the unaided or naked eye. This makes it possibly the most interesting object for observation.

The Moon is very bright, with an apparent magnitude of -12.74 (this is all light from the Sun reflecting off its surface – keep that in mind for later). It is also a relatively large  satellite compared to its planet, the Earth. It is the fifth largest natural satellite in the Solar System (the other big ones all orbit gas giants). The diameter of the Moon is more than one quarter the size of the Earth’s.

Put this together, and you get that the Moon is easy to see, but that only half of it is lit at a time. One more piece to add in.

The Moon is “tidally locked” with respect to the Earth. This means that we always see the same side of the Moon from the surface of the Earth (technically, it wags from side to side slightly, a process known as “libration”, and so about 59% can be seen at different times in its orbit). The tidal locking is because the Moon’s rotation about its axis and its revolution around the Earth both take about the same amount of time, 29.5 days.

It is accurate to refer to the near side or far side of the Moon – but not astronomically correct to speak of a “light side” or “dark side”, unless you are strictly speaking from the perspective of someone on the Moon. The phases of the Moon occur as the lighted area of the Moon, that which faces the Sun, moves across the near side. We see the entire lighted portion during a Full Moon, and none during the New Moon.

The Moon’s orbit is tilted about 5 degrees from Earth’s orbit around the Sun. It is only when the orbits line up precisely that eclipses occur: a lunar eclipse during a Full Moon, and a solar eclipse during the New Moon. The solar eclipse is possible only because the apparent size of the Sun and Moon are about the same, about one half degree. Space probes have observed eclipses where this was not the case..

During a lunar eclipse, the Moon passes through the shadow of the Earth, and during the period of totality, the only sunlight falling on the Moon is that which is passing through the Earth’s atmosphere. The same bending of light rays that produces the reds of sunrises and sunsets also causes this light to turn red, as we just saw a few days ago on the eclipse on January 20th (if you were lucky enough to be able to see it, as I was not).

The entire phase cycle of the Moon takes 29 1/2 days. About 7 days after each New Moon there will be a First Quarter, followed by a Full Moon a week later. During this period it is said to be “waxing” or the lighted portion as seen from Earth is getting bigger. After the Full Moon it is “waning” or getting smaller until the next New Moon. When the phase is between a Quarter and Full Moon its shape is called “gibbous”. When it is between Quarter and New Moon, we call the shape a crescent.

One more detail about the appearance during phases, and this is dependent on which hemisphere you’re in. Here in Milton, in the Northern Hemisphere, the light moves from right to left. At first, we will see the dark portion of the phase on the left (the East) when the Moon is waxing. Later when it is waning, the right side will be dark. (This direction is opposite south of the equator, where the Moon and stars appear 180 degrees inverted or rotated).

I will cover more detailed observation of the Moon in a subsequent article, with topics such as the craters and seas of the Near side, and the names they are called by.

Happy Moon-watching!


Wikipedia. (2019, January 25). Lunar phase. Retrieved from https://en.wikipedia.org/wiki/Lunar_phase.

Wikipedia. (2019, January 25). Moon. Retrieved from https://en.wikipedia.org/wiki/Moon.

Absolute and Apparent Magnitude: Measures of Brightness

By Peter Forrester | January 17, 2019

I’ve sometimes referred to the brightness of different stars and other objects in the sky. Let me tell you how astronomers measure brightness.

The first term you need to know is “apparent magnitude,” which is just a fancy way of saying how bright an object appears to be, usually when seen from the surface of the Earth.

The second term is “absolute magnitude”, which means if you put the stars at the same distance from Earth, what would the brightness be then?

Now let me explain about the numbering scale used to express these brightness. This is where it gets to seeming crazy. You see, the brighter an object is, the lower the number is. For instance, the brightest object, the Sun, has an apparent magnitude of -26.74. The next brightest star, Sirius, comes in at “only” -1.46.

The peculiar scale goes back to the Greek writer Hipparchus in the 1st century BC. He labelled the brightest 20 stars as first degree stars, and the dimmest as 6th degree. This was a rather simple way of describing brightness, before there were telescopes or instruments for measuring brightness precisely. Stars that are too dim to see with the naked eye are 7th degree or lower.

Eventually, after measurement of magnitude started, a decimal form started to be written. This system was formalized in 1856 by an English astronomer named Norman Robert Pogson (1829-1891).  Under his system, a first degree star is 100 times as bright as a sixth degree star, and so each degree represents a ratio of about 2.5, sometimes called “Pogson’s ratio”. For you math nerds, the exact amount is the 5th root of 100.

Pogson’s system assigned the North Star, Polaris, as being of degree 2.0; however this was later changed because Polaris’ magnitude varies slightly over time. The star Vega is now defined as 0.00 magnitude. There are four stars brighter than Vega, which necessarily means they have negative numbers in their magnitude.

Here is a list of the 10 brightest objects in the sky (the planets and Moon are listed at their brightest but they vary over time). It should be noted that there are different possible ways of measuring apparent magnitude of stars, and you may see these in a slightly different order. See below Wikipedia, List of brightest stars for more information on these variations.

  1. The Sun: -26.74
  2. The Moon: -12.74
  3. Venus: -4.89
  4. Jupiter: -2.94
  5. Mars: -2.91
  6. Mercury: -2.45
  7. Sirius: -1.46 (star in Canis Major)
  8. Canopus: -0.74 (star in Carina*)
  9. Saturn: -0.49
  10. Rigil Kentaurus: -0.27 (star in Centaurus*)

*Note: Carina and Centaurus can only be seen from the Southern Hemisphere.

Now to Absolute Magnitude. This is defined as the brightness an object would have if seen from a standard distance (10 parsecs, or 32.6 light-years), adjusting for interstellar dust. They can also measure it in different light bands, but I won’t bore you with an explanation of that. See Wikipedia, Absolute magnitude for more on this. Warning: there’s a lot of complicated math on this page. Also, note that a parsec is a measure of distance, not time as implied in the first Star Wars movie.

Measurement of absolute magnitude is made with an instrument called a bolometer, and varies based on what type of light wavelength you’re looking at.

Some stars are so bright that they would appear brighter than the planets and cast shadows if they were only 10 parsecs away. For example, Rigel is -7.0, Deneb is -7.2, and Betelgeuse in Orion has an absolute magnitude of -5.6. By comparison, Sirius is 1.4, much brighter than the Sun’s absolute magnitude 4.83.

Apparent magnitude for objects in the solar system is based on supposing that the object were a standard distance of 1 Astronomical Unit (about 93 million miles, or the distance between the Sun and Earth) from both the Sun and the observer.

So now you know the difference between apparent and absolute magnitude. Now if someone asks how to measure the brightness of stars, you’ll know the answer!


Wikipedia. (2019, January 14). Absolute magnitude. Retrieved from https://en.wikipedia.org/wiki/Absolute_magnitude.

Wikipedia. (2019, January 11). Apparent magnitude. Retrieved from https://en.wikipedia.org/wiki/Apparent_magnitude.

Wikipedia. (2018, 24 October). First magnitude star. Retrieved from https://en.wikipedia.org/wiki/First_magnitude_star.

Wikipedia. (2019, January 1). List of brightest stars. Retrieved from https://en.wikipedia.org/wiki/List_of_brightest_stars.

Wikipedia. (2018, 23 July). N. R. Pogson. Retrieved from https://en.wikipedia.org/wiki/N._R._Pogson.

Skies over Milton, January Edition

By Peter Forrester | January 3, 2019

Greetings, everyone!

Here are your stargazing news and events for the month of January.

Thursday, January 3: Quadrantid Meteor Shower, active from Dec 28 to January 12, peaks tonight at 9:00 PM. Up to 120 meteors per hour. The radiant (location in sky where meteors appear to originate) is in the constellation Bootes.

Saturday, January 5: New Moon at 8:28 PM. Also partial eclipse of the Sun visible in northeast Asia and the North Pacific. Unfortunately we can’t see it here as the Sun has already set.

Tuesday, January 8: Moon at apogee (furthest from Earth) at 11:00 PM.

Saturday, January 12: Moon near Mars in the evening, 8:00 PM.

Monday, January 14: First Quarter Moon at 1:45 AM.

Monday, January 21: Total eclipse of the Moon, this one is visible here from 11:41 PM on January 20 until 12:43 AM on the 21st. Mid-eclipse at 12:31 AM. Partial phases start at 10:34 PM and end at 1:51 AM . Moon at perigee (closest to Earth, this one is considered a Supermoon). A lunar eclipse is always the Full Moon as well.

Tuesday, January 22: Venus near Jupiter at 11:00 AM, though we’ll have to wait until after sunset to see this spectacle. Also that same day, Moon near Regulus (bright star in the constellation Leo) at 11:00 PM.

Sunday, January 27: Last Quarter Moon at 4:00 PM.

Wednesday, January 30: Moon near Jupiter at 9:00 PM.

Here’s wishing you all a happy month of skywatching! For other events, and more stargazing tips, check out the link to skymaps.com.

Previous in series: Skies Over Milton, December Edition

| Next in series: Skies Over Milton, February Edition

See also: peterforresterimages.wordpress.com, where I will be posting images from a planetarium program I am now using. I just uploaded images for the articles on Orion and Venus.


Thalassoudis, Kym. (2000-18). Skymaps. Retrieved January 3, 2019 from www.skymaps.com.

Wikipedia. (2019, January 3). January 2019 Lunar Eclipse. Retrieved from https://en.wikipedia.org/wiki/January_2019_lunar_eclipse.

Wikipedia. (2019, January 3). Quadrantids. Retrieved from https://en.wikipedia.org/wiki/Quadrantids.

Wikipedia. (2019, January 3). Regulus. Retrieved from https://en.wikipedia.org/wiki/Regulus.

Wikipedia. (2019, January 3). Solar Eclipse of January 6, 2019. Retrieved from https://en.wikipedia.org/wiki/Solar_eclipse_of_January_6,_2019.

Constellation of the Month: Orion

December 31, 2018 | By Peter Forrester

We’ve had several clear or partially clear nights recently. If you’ve looked up just slightly while facing east about 6 or 7 pm, chances are you’ve seen the Constellation of the Month.

Orion is one of the brightest and best-known constellations in the sky. Even people who know very little about astronomy or stars may recognize this particular shape in the sky. It is located very close to several of the “zodiac” constellations. Due to its brightness, and familiarity, it can be used to find many other constellations. Thus it seems an appropriate choice for the first constellation of the month.

Orion is sometimes called a winter constellation. In most of the summer it is not visible in the Northern Hemisphere, and at other times it can only be seen in the early morning. I recommend dressing up and braving all this cold weather to get a good look at this thing; it really is the best time of year to see it. It’s so bright, I can catch glimpses of it while driving home in the evening (not that you should be looking at stars behind the wheel, of course).

Orion has been pictured as a hunter since the time of the early Greeks. The seven brightest stars form a shape like an hourglass. Four make up his shoulders and feet, and the other three are his belt. He is holding something in front of him, interpreted as a shield. There are also many dimmer stars, and several nebulae (clouds of dust and gas out in space) which can be seen with the naked eye. Looking at these objects through binoculars is an even more wonderful experience.

To the left of Orion are found the two dogs, Canis Minor and Canis Major, and above him to the right, on the other side of the shield, is Taurus the Bull. Above him and a bit to the left is the constellation Gemini, the twins. Taurus and Gemini you might recognize as the names of two zodiac constellations.

Rigel is usually the brightest star in Orion, and the seventh brightest star in the night sky. I say “usually” because the star Betelgeuse (pronounced BEETLE-juice) is a variable star, and is sometimes brighter than Rigel. Rigel is considered to be the left foot of Orion, though it appears on the right side from our perspective. The second and third brightest stars in Orion, the two shoulders, are called Betelgeuse and Bellatrix. They have both had their names borrowed for popular works of fiction. For some reason Rigel’s name has not been as popular. The other bright stars (among the 7 brightest) include Saiph, the other foot, and the three in the belt are called Alnitak, Alnilam, and Mintaka. Sidenote: many star names come from Arabic, perhaps because of an early Muslim astronomer (Abd al-Rahman al-Sufi) who made detailed drawings and descriptions in his book, called Book of Fixed Stars, published around 964 A.D.

I am sure many readers are interested in knowing how you can use Orion to locate other constellations. There are many good illustrations of this online. A simple search will bring up many of these. My favorite drawings that I’ve found so far are found on a site of educational articles called Owlcation; see the first reference below. The article has some typos but the drawings are great. The author also recommends two free astronomy software programs, which I plan to review in the near future.

Following the line of Orion’s belt down and to the East (left), you come to Sirius, the brightest star in the night sky, which is part of the large dog, Canis Major. Following the belt the other way leads you to Aldebaran, the brightest star in Taurus. You can also draw a line from the bottom right star, Rigel, up through the top left, Betelgeuse, and this leads you to Castor and Pollux, the two brightest stars in Gemini. And a line through the shoulders, down and to the East leads you to Procyon, the brightest star in the small dog, Canis Minor.

Besides Taurus, Gemini, and the two dogs, you can also locate the Pleiades star cluster, as well as a constellation called Cetus, the Whale by drawing lines using stars in Orion. There are also two patterns of stars or asterisms around and including Orion that can be used to locate various constellations. The Winter Triangle contains Sirius, Procyon and Orion’s Betelgeuse. The larger Winter Circle or Hexagon is composed of six stars, one of which is Rigel.

Orion has had many different names in different ancient nations’ descriptions and mythology. It has been identified with the Egyptian god Sah, and with the founder of the Armenian nation, Hayk. It has not always been seen as a hunter, or even as a man. There is much more of the mythology and history to be found in the Wikipedia article, including a Greek story about why Orion and the constellation Scorpio are never in the sky at the same time.

People in both the Northern and Southern Hemispheres can see Orion in the early evening right now, though it is summer for the people in the Southern Hemisphere. It also appears upside down there, from my northern perspective in Milton, and above rather than below the ecliptic. It is almost overhead for people near the Equator, however, and not visible at all at the South Pole, because the Sun doesn’t set during summer there.

See also: Observing the Planets: Venus | What Is the Zodiac, Anyway? | Skies Over Milton, December Edition


Owlcation. (Updated 2018, March 13). Using Orion to find Stars and Constellations. By RaulP. Retrieved from https://owlcation.com/stem/Using-Orion-to-find-Stars-and-Constellations-part-1

Wikipedia. (2018, December 31). Book of Fixed Stars. Retrieved from https://en.wikipedia.org/wiki/Book_of_Fixed_Stars.

Wikipedia. (2018, December 31). List of Arabic Star Names. Retrieved from https://en.wikipedia.org/wiki/List_of_Arabic_star_names.

Wikipedia. (2018, December 31). Orion (constellation). Retrieved from https://en.wikipedia.org/wiki/Orion_(constellation).

Wikipedia. (2018, December 31). Winter Hexagon. Retrieved from https://en.wikipedia.org/wiki/Winter_Hexagon.

Observing the Planets: Venus

By Peter Forrester | December 26, 2018

If you have gotten out to look at the sky one or two hours before sunrise, and seen a really bright star to the east or slightly southeast, that is actually not a star, but the planet Venus.

The second planet from the Sun is the third brightest object in the sky, after the Sun and Moon. It outshines even the brightest stars, with its apparent magnitude reaching as high as -4.6 (for historical reasons, the brightness index is inverted, with the lowest numbers being the brightest).

This neighboring planet is a twin of the Earth – they are very similar in size, though Venus’s thick atmosphere of mostly carbon dioxide would be poisonous to breathe. That atmosphere, however, is a boon for stargazers, as it reflects sunlight very well.

Venus is often called the “morning and evening star” because it appears just before sunrise or just after sunset. Since it is closer to the Sun than our Earth, which in astronomers’ language makes it an “inferior planet”, it is always fairly close to the Sun in the sky, within about 47 degrees. Because it is closer to the Sun, Venus also goes through phases, like the Moon, which can be seen through a strong pair of binoculars, or a telescope. It is at its brightest when in crescent phase, ironically enough, because that is when it is closest to us.

The best time to see Venus is either in the early morning, 1-2 hours before sunrise, or 1-2 hours after sunset, though it can also be seen sometimes during full daylight. Right now it is in its “morning” part of its cycle. The full cycle of morning star and evening star (including times when it can not be seen at all) takes 584 days, although Venus only takes 224 days to orbit the Sun.

Occasionally Venus crosses in front of, or “transits” the Sun. The last time this happened was in 2012. I would caution you to take extreme care when watching this, using undamaged eclipse glasses (anything not so dark, such as ordinary sunglasses, exposes your eyes to damage from the Sun). But it seems pointless – the next transit of Venus will be in 99 years, in the year 2117. However, the smaller planet of Mercury transits much more often; more on this later.

Being a bright planet, Venus has been known ever since man stood and looked up at the stars, though apparently it took the ancients a while to discover that the “morning star” and the “evening star” were the same object. The oldest records show the ancient Sumerians recognized it as one object, connecting it with their goddess Inanna; however the Chinese had separate names for the morning and evening appearances. The Babylonians also had detailed observations and called it “the bright queen of the sky”. The Romans called the two aspects Lucifer, the “Light-bringer”, and Vesper, the name of the evening star (Hesperus was the Greek equivalent, and considered to be a god), though the Greeks had discovered that they were one object (this rediscovery was credited to both Pythagoras and Parmenides). Of course, famously, the Romans also named the planet after their goddess of love.

The Pawnee in North America and the Mayans also observed Venus. In fact, measuring the movement of the planet was one of the main reasons the Mayans built their observatory at Chichen Itza, and some parts of their calendar system are based on the movements of Venus.

As late as 1960, writers were hoping its thick atmosphere would make the planet conducive to life, but these hopes were shattered by the various space probe missions to the planet, the first of which was sent in 1961. NASA’s Mariner 2 was the first successful flyby in 1962, later followed by several successful missions from the Soviet Union. These Russian probes became the first to accomplish several things in space exploration.

  • The first to enter the atmosphere of another planet: Venera 4, 1967
  • The first to make a soft landing on another planet: Venera 7, 1970
  • The first to return photographs from the surface of another planet: Venera 9, 1975
  • The first to perform high-resolution radar mapping of Venus: Venera 15, 1983

These observations, as well as improved technology of Earth-based telescopes, helped clinch the death of the idea of life on Venus. In addition to the high CO2 content, it also has a very high atmospheric pressure, 100 times that of Earth, and the surface temperature of 860 – 900 degrees Fahrenheit is hot enough to melt lead. Apparently there is still hope of doing missions to the upper atmosphere, however, where the temperature and pressure are much lower.

For more on the history and observation of this sister of our Earth, see the sources below. In particular, the Cosmic Pursuits article has an excellent explanation and diagram of the phases of Venus.

Happy stargazing!

See also:

Skies Over Milton, December Edition and What Is the Zodiac, Anyway?


Cosmic Pursuits. (2016, November 24). A Brief Guide to Observing the Planet Venus. By Brian Ventrudo. Retrieved from https://cosmicpursuits.com/1366/guide-to-observing-planet-venus/

Wikipedia. (2018, December 26). Transit of Venus. Retrieved from https://en.wikipedia.org/wiki/Transit_of_Venus.

Wikipedia. (2018, December 26). Venera. Retrieved from https://en.wikipedia.org/wiki/Venera.

Wikipedia. (2018, December 26). Venus. Retrieved from https://en.wikipedia.org/wiki/Venus



What Is the Zodiac, Anyway?

By Peter Forrester |  December 20, 2018

Many people know the term “zodiac” from astrology, or fortune-telling based on stars and planets. But how many know the astronomical, scientific use of the word?

First of all, I need to introduce another term, the “ecliptic”. This can be imagined as an invisible line in the sky through which the Sun appears to move every year. I use the word “appears” because we are on a spinning Earth, which is the real source of the Sun moving across the sky every day. The Moon and planets always stay very close to this line. The ecliptic gets its name because eclipses only occur when the Moon is crossing it during full moon or new moon.

The ecliptic is inclined 23.4 degrees from the “celestial equator”, the line in the sky lying directly over the Earth’s equator. This corresponds to the well-known tilt in the Earth’s axis, in its orbit around the Sun.

From our vantage point in the Northern Hemisphere, the ecliptic rises from the horizon in the east, reaches its highest point to the south, and then descends to the west. So if you want to see a zodiac constellation, or a planet, or the Moon, your best bet is to start by looking south.

The concept behind assigning a Zodiac constellation to each month is a rough approximation of the scientific reality. The idea is that every month, the Sun occupies the space between the Earth and one of the twelve classic Zodiac constellations, and hence is said to be “located” in that constellation. Here the zodiac uses 12 divisions of the ecliptic into 30 degrees each, regardless of how big each of these constellations actually is.

However there are a couple of little catches here. There are actually 13 constellations through which the ecliptic passes. The one not used in the common 12-sign version of astrology is called Ophiuchus, the Serpent Bearer, and is located between and above Sagittarius and Scorpius. The constellations are different sizes and thus the astrological divisions reflect simplifications of where the Sun and planets are.

A further problem with the signs of the Zodiac is that they do not align properly with the current position of the Sun. For example, the astrological sign Aries is from March 21 to April 20, but the Sun actually passes in front of Aries between late April and mid-May. This is because of long-term fluctuations in the orbit and rotation of the Earth. The dates of the signs were assigned over 2000 years ago, and now are out of alignment.

I need say very little about the third major problem with astrology in general. There is no scientific evidence that a person’s life or personality, or future prospects have anything to do with the position of the stars or planets at the time of their birth, or at any other time. Astrology is a pseudo-science, a cool belief system that has very little basis in reality. I am here to explain the reality to you, as harsh as it may sound.

The zodiac, as used in astronomy, is a small band in the sky, about 8 degrees above and below the ecliptic, in which the Sun, Moon, and Solar system planets are usually located (there are as many as 10 other constellations in which planets can occasionally be found). Besides the 13 constellations mentioned above, which all cross the line of the ecliptic, there is a 14th one that nearly touches it and which the solar system objects more commonly occupy. This constellation is called Cetus, the Whale, and lies just below Pisces.

I’ll discuss the various zodiac constellations later, but for now I just want to list them, with the dates that the Sun is actually located there. You will notice these dates are about a month later than the traditional astrological  dates, which I will not give here. You can easily find them if you want to.

Aries: April 18 – May 13

Taurus: May 13 – June 21

Gemini: June 21 – July 20

Cancer: July 20 – August 10

Leo: August 10 – September 16

Virgo: September 16 – October 30

Libra: October 30 – November 23

Scorpio: November 23 – November 29

Ophiuchus: November 29 – December 17

Sagittarius: December 17 – January 21

Capricorn: January 20 – February 16

Aquarius: February 16 – March 11

Pisces: March 11 – April 18

Just one further point – it will be difficult to see the constellation in the month of its “sign”, since it is too close to the Sun, which means the sky will be too bright to see it when it is up.

Some of the zodiacs are dim, and others are very bright. We’ll get into the easiest ones to see on another occasion.

For now, your friendly Milton astronomer signing off.

See also: Skies Over Milton, December Edition.


Wikipedia. (2018, December 20). Cetus. Retrieved from https://en.wikipedia.org/wiki/Cetus.

Wikipedia. (2018, December 20). Ecliptic. Retrieved from https://en.wikipedia.org/wiki/Ecliptic#Plane_of_the_Solar_System.

Wikipedia. (2018, December 20). Ophiuchus. Retrieved fromhttps://en.wikipedia.org/wiki/Ophiuchus

Wikipedia. (2018, December 20). Sidereal and Tropical Astrology. Retrieved from https://en.wikipedia.org/wiki/Sidereal_and_tropical_astrology.

Wikipedia. (2018, December 20). Zodiac. Retrieved from https://en.wikipedia.org/wiki/Zodiac.

Skies Over Milton, December Edition

By Peter Forrester | December 6, 2018

Greetings, stargazers!

It will be an exciting month for night skywatchers.

Here are some significant events to look for this month, provided that cloudy weather doesn’t get in the way. I’m sure all you hardy ones won’t mind the cold if it means getting a look at some really important objects in the sky.

First, we have a New Moon on Friday, December 7th. This time of the lunar cycle is an ideal opportunity for stargazers because it means you can see dimmer stars without the greater light of the moon overshadowing them, so to speak. This is the point of the moon’s orbit around the Earth when it is almost directly between the Earth and the Sun, and therefore we see only its unlighted side (when it gets directly between, we get a solar eclipse, either total or partial, these only occur between 2 to 3 times a year).

Second, on Friday, December 14th, we get the peak of the Geminid meteor shower. Shooting stars will be visible each night between Friday, the 7th and Sunday, the 16th, with the largest number (up to 80 per hour) occurring on the peak night. The meteor shower occurs every year when the Earth’s orbit enters debris left by an asteroid, 3200 Phaethon. It is called “Geminid” because the origin of the shooting stars appears to be in the constellation Gemini, located just above the prominent constellation Orion. However, the shooting stars can be seen in any part of the sky. There are usually only a few shooting stars in the early evening, and the best time to see them is around 2 am for you hardcore stargazers!

Third, occurring about the same time we have the brightest comet flyby of the year. Comet 46P/Wirtanen will pass only about 7.2 million miles away, among the 10 closest approaches by comets to the Earth in modern times. It may be bright enough to see with the naked eye, especially in areas without a lot of light pollution, which I have not generally found to be a problem here in Milton. The comet will not be sharp and contained in a single point like a star, but will be more spread out. The comet will be closest to Earth, and therefore appear the largest, on Sunday, December 16th, between 9:30 and 10 pm on the East Coast. At this time it will pass very near the star cluster the Pleiades.

The comet circles out to near the planet Jupiter and back, orbiting every 5.4 years around the sun (perihelion – closest approach to the Sun is on Wednesday, December 12th), and was first discovered in 1948, by an American astronomer named Carl A Wirtanen. Don’t worry, its orbit is well known by astronomers, and there is no chance of it hitting the Earth! For more information on the campaign to observe the comet, see http://www.wirtanen.astro.umd.edu.

Fourth, we have a Full Moon on Saturday, the 22nd. For your information, the quarter moons occur on Saturday, the 15th (First Quarter), and Saturday, the 29th (Last Quarter).

There are also many events concerning when the Moon is closest to a particular star, or constellation, or when a planet is to be seen nearest to a bright star. For more information of this type, see http://skymaps.com. They publish a free monthly star chart, with 3 versions for different latitudes. I use the one for 40 degrees north of the Equator (Milton is above 43 degrees North). This chart is also useful up to 15 degrees north and south of that line. They also publish versions for the Equator, and for the Southern Hemisphere. On the second page of the sky chart is information about objects that can be seen either with the naked eye, with binoculars, or with a telescope.

Fifth, there is another meteor shower, whose peak night is also on Saturday, the 22nd. This shower has slower-moving meteors (compared to the Geminids earlier in the month) that radiate from a point in Ursa Minor (also known as the Little Dipper). It starts around Monday, the 17th and runs until the 26th of the month. The debris field causing this annual shower is associated with a comet called 8P/Tuttle. Like the other meteor shower, the best time to see it is after midnight, and due to its location it is always visible to the North.

Next in series: Skies over Milton, January Edition


CBS News. (2018, December 5). Brightest comet of the year will zoom near Earth next week). By Caitlin O’Kane. Retrieved from https://www.cbsnews.com/news/brightest-comet-of-2018-46p-wirtanen-will-zoom-near-earth-next-week/

Thalassoudis, Kym. (2000-17). Skymaps. Retrieved December 6, 2018 from Skymaps.com.

Wikipedia. (2018, December 6). 46P/Wirtanen. Retrieved from https://en.wikipedia.org/wiki/46P%2FWirtanen

Wikipedia. (2018, December 6). Geminids. Retrieved from https://en.wikipedia.org/wiki/Geminids

Wikipedia. (2018, December 6). New Moon. Retrieved from https://en.wikipedia.org/wiki/New_moon.

Wikipedia. (2018, December 6). Perihelion and aphelion. Retrieved from https://en.wikipedia.org/wiki/Perihelion_and_aphelion

Wikipedia. (2018, December 6). Ursids. Retrieved from https://en.wikipedia.org/wiki/Ursids

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