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.

Every Watch Is a Compass

By Muriel Bristol (Transcriber)  | December 20, 2018

Don’t get lost:

Every Watch Is a Compass

A few days ago I was standing by an American gentleman, when I expressed a wish to know which point was the north. He at once pulled out his watch, looked at it, and pointed to the north. I asked him whether he had a compass attached to his watch. “All watches,” he replied,  “are compasses.”

Then he explained to me how this was. Point the hour hand to the sun and the south is exactly half-way between the hour and the figure XII on the watch. For instance, suppose that it is 4 o’clock. Point the band indicating four to the sun and II on the watch is exactly south.

Suppose that it is 8 o’clock, point the band indicating eight to the sun and the figure X on the watch is due south. My American friend was quite surprised that I did not know this.

Thinking that very possibly I was ignorant of a thing that everyone else knew, and happening to meet Mr. Stanley, I asked that eminent traveler whether he was aware of this simple mode of discovering the points of the compass. He said that he had never heard of it. I presume, therefore, that the world is in the same state of ignorance.

Amalfi is proud of having been the home of the inventor of the compass. I do not know what town boasts of my American friend as a citizen. – London Truth (Vermont Journal (Windsor, VT), November 1, 1890).

Dr. Livingston, I presume?


Ordnance Survey. (2011, August 22). Forgotten Your Compass? Use the Sun to Navigate. Retrieved from www.ordnancesurvey.co.uk/blog/2011/08/forgotten-your-compass-use-the-sun-to-navigate/

Wikipedia. (2018, December 2). Flavio Gioja. Retrieved from en.wikipedia.org/wiki/Flavio_Gioja

Wikipedia. (2018, December 20). Henry Morton Stanley. Retrieved from en.wikipedia.org/wiki/Henry_Morton_Stanley

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