More Facts About Jupiter

By Peter Forrester | March 29, 2019

I previously wrote about observation of the planet Jupiter. I thought as a bonus, some of you might like to read some more about the fifth planet from the Sun.

Jupiter is the largest of the eight planets in our Solar System. It is named after the king of the gods in Roman mythology. It is made up mostly of gases, and has a prominent storm called the Great Red Spot that has persisted for hundreds of years and is bigger than the Earth. But everyone knows these facts, right? What about some facts about the Red-spot planet that you might not know about?

Some other interesting facts about the largest planet:

Jupiter is mostly made of the lightest element, hydrogen, although about 25% of its mass is helium, the second element (only about 10% of the molecules).

The first spacecraft to fly near Jupiter was called Pioneer 10, in 1973. Pioneer 11 then flew even closer in 1974, followed by the Voyager 1 and 2, both of which flew further away in 1979 but used its gravity to speed them up on the way to the more distant planets.

Jupiter is currently being orbited by a space orbiter called Juno, which entered into orbit in 2016 and will continue until 2021, when it will be intentionally steered into the planet’s outer gas layers (to protect the moons from a collision).

This is the second spaceship to orbit the king of the planets – the previous one, Galileo, orbited from 1995 to 2003, and is named after the Italian astronomer Galileo Galilei, who improved on a recent (1608) invention: the refracting telescope, and is one of the first people known to have used it for astronomy.

Through his telescope in January 1610 he discovered the four largest moons of Jupiter, the first group of objects ever to be discovered that were definitely orbiting an object other than the Sun or Earth. The discovery of the Galilean moons provided evidence to Galileo that all objects in the solar system do not orbit the Earth (this evidence spelled the doom of the Geocentric model of the solar system, although it took a while for the theory to be fully abandoned by scientists). I will let you read for yourself of the religious controversy this caused, which led to Galilei being obliged to recant any belief that the Earth is not the center of the universe, and also spending the end of his life under house arrest.

For a long time Galileo was credited as the sole discoverer of these moons, and hence they are called the Galilean moons after him (though they were also independently discovered by Simon Marius in Germany, a month or two before that, who proposed their individual names when he published his findings in 1614). Galileo also made the first telescopic observations of several other amazing things: craters on the Earth’s Moon, analysis of sunspots, and the phases of Venus, though the telescope only magnified 30 times, much less than the powerful ones used now. He also may have seen the planet Uranus in 1612 (discovered to be a planet in 1781), and also wrote on physics and engineering.

The Galilean moons are called Callisto, Ganymede, Europa, and Io (all named after mythological lovers of the god Jupiter). Ganymede is the largest moon and 9th largest object in the Solar System, being larger than the planet Mercury. It is also the largest object in the Solar System that does not have a substantial atmosphere. Three of the four moons are larger than our Moon (all except Europa). All four are also rounded by their own weight unlike the smaller ones, and hence would be considered “dwarf planets” if they orbited the Sun.

The space mission called Galileo made some amazing discoveries, mostly about Jupiter and its moons. For example, it discovered evidence of a liquid ocean under the surface of the moon Europa, now widely considered to be the most likely place in the solar system, outside of Earth, for life to still exist (though it is likely it once existed on Mars). It also found evidence to explain the origins of Jupiter’s thin rings (much less visible than Saturn’s famous ones).

On its way to Jupiter, the probe also observed the collision of comet fragments into Jupiter in 1994 (the first impacts were not visible from Earth), as well as discovering the first known asteroid-moon system in 1993.

Speaking of moons, there are now 79 known moons of Jupiter with stable orbits, the most satellites discovered around a single object to date other than the Sun. The four Galilean ones are much bigger than all the rest of Jupiter’s moons. No more moons for Jupiter were discovered until 1892. This moon, Amalthea’s largest dimension of 250 km is less than one tenth the diameter of Europa, the smallest of the Galilean moons. Amalthea was the last planetary moon to be discovered by direct visual observation.

If Jupiter was 75 times more massive, it would be capable of nuclear fusion and be considered a star, and give off light. However, if it got just a little more massive than it is now, it is thought that it would start shrinking and become denser (in other words, its radius would actually decrease). However, it gives off more heat than it receives from the Sun, and actually does shrink by about 2 cm per year. Jupiter has 2 and a half times the mass of the other 7 planets combined. Its mass is often used as a comparison when talking about extrasolar planets that have been discovered.

It is theorized but unknown whether Jupiter has a solid core.

Jupiter could fit 10 times across the diameter of the Sun.

Every 398 days Earth overtakes Jupiter in its orbit, and then it appears to move backwards for a few days.

Jupiter rotates about every 10 hours (the fastest of any planet in the Solar System). The cloud layers at the poles orbit at a different rate than those at the equator of Jupiter, about 5 minutes longer, although the planet’s official rotation rate is based on measurements of its magnetosphere by radio waves.

I could go on and on, but you can all read as much of this as you want over on Wikipedia or Google other sources. Until next time, adieu!


Wikipedia. (2019, March 15).  Comet Shoemaker-Levy 9. Retrieved from .

Wikipedia. (2019, March 13). Galileo (spacecraft). Retrieved from

Wikipedia. (2019, March 18). Galileo Galilei. Retrieved from

Wikipedia. (2019, March 22). Juno (spacecraft). Retrieved from

Wikipedia. (2019, March 26). Jupiter. Retrieved from

Observing the Planets: Jupiter

By Peter Forrester | March 28, 2019

Jupiter, as seen from the surface of the Earth, is on average the second brightest of the five visible planets. Only Venus is brighter (and sometimes Mars). This is reflected light from the Sun, as planets don’t produce any light of their own. Jupiter’s reflected light is bright enough to cast shadows on Earth (the brightest apparent magnitude it reaches is -2.94).

Being so bright, Jupiter has been known since ancient times, and was thought to represent the god Marduk to the Babylonians, and the gods Zeus and Jupiter in Greek and Roman mythology, the last of whom it is named for.

Unlike Venus and the Moon, Jupiter doesn’t go through a full range of phases, being more distant from the Sun (at times as much as 11.5% of the side we see is not illuminated, so it does have the Full and Gibbous phases only). It also takes 11.86 years to orbit the Sun, meaning it will move about 30 degrees in the sky every year (the twelve years of the Chinese calendar were originally based on this orbital period). So it can be hard to remember where it is located, but it (and even more so the more distant planets) tend to move very slowly among the stars, lingering in the same constellation for months to years. This is an advantage if you can remember where to look. Jupiter will take many months to cross Ophiuchus, since it is a very large constellation.

We just passed an occasion a couple of days ago where the Moon was located very close to Jupiter. But how can you find it normally?

Jupiter has just entered the constellation Ophiuchus, the “13th zodiac” constellation, located above Scorpius. According to the planet locator on (see link below), it will be above the horizon (from their nearest location to Milton in Concord, New Hampshire) between 1:24 and 10:25 am tomorrow.

If you have a smartphone, you can also find the current location of any planet on the free app Sky Map (you have to make sure the constellation labels are on and follow the Zodiac constellations around to find all the planets, Moon, and Sun). You can find some other excellent websites by doing a simple Google search.

Observing Jupiter with the naked eye, it just looks like a very bright yellowish star. However, if you have a chance to look at it in a telescope, which I have had the pleasure to do, I suggest you take it. You can see its round shape, including the aforementioned Great Red Spot, as well as the many belts of clouds and some or all of the four Galilean moons, whichever happen to be on the same side. The whole planet will be lit up because we are between Jupiter and the Sun right now.

Stay tuned for more on this amazing object in the sky. Have fun watching Jupiter, and pleasant star-gazing!

Previous in series: Observing the Planets: Venus


Time and Date A.S. (1995-2019). Night Sky Map & Planets Visible Tonight. Retrieved from

Wikipedia. (2019, March 26). Jupiter. Retrieved from

Observing the Moon: Part 2, Just What is a Super-Moon Anyway?

By Peter Forrester | March 20, 2019

Well, here we go. Another supermoon is coming up tonight / tomorrow (depending on where you are), right after the Spring Equinox which is tonight about 6 pm in US Eastern time. This one is called the Super Worm Equinox Moon, or similar phrases. What exactly do all these terms mean?

Well, first of all, “supermoon” means a Moon that is at its closest approach to Earth, and at full moon (or new moon) phase at the same time, making it appear brighter and slightly larger than it normally would. Every month it has its “perigee”, the closest approach of that orbit, and the opposite “apogee”, but some perigees are closer than others. The moon at perigee normally appears about 14% bigger than at apogee, so we’re not talking a huge difference here.

The term supermoon was first coined by an astrologer in 1979, and has various definitions in different places. The original definition stated that the full or new moon occurs with the Moon within 90% of its closest approach to Earth, but the reason for choosing 90% was never explained, and the claim that a supermoon causes catastrophic effects and stress on Earth has been thoroughly disproven by scientists. It should also be noted that most people using the term “supermoon” are referring to a Full Moon (and very rarely is used for a New Moon).

The Worm refers to the month of March, and apparently this originates with Native Americans who called their full moon in March by this name, owing to earthworms coming out of the thawing soil at this time of year.

Equinox, well that’s happening today. We only have two equinoxes per year, and they always occur around March 20th, and again 6 months later in September. For the Northern Hemisphere, the March equinox is called the Vernal or Spring Equinox, and denotes the astronomical beginning of Spring, while it starts Fall in the Southern Hemisphere. The September equinox is called the Autumnal or Fall Equinox.

But what is an Equinox? According to Wikipedia, equinox occurs at “the instant of time when the plane (extended indefinitely in all directions) of Earth’s equator passes through the center of the Sun”. Or in other words, when the center of the Sun is directly above the equator.

The two equinoxes are when the day and night are the same length (not exactly at the same moment as the Equinox, but pretty close). The equinoxes are also the only times of year that both the northern and southern hemisphere receive the same amount of light from the Sun.

This March Supermoon is the last full moon supermoon of this year, after having them in January and February as well (however, we have three supermoon New moons in August and September). The February one was the closest and brightest, but this one is unique. It is very rare for the supermoon to happen within a day of the equinox. It won’t happen again until 2030.

Owing to the momentous occasion of a Super Worm Equinox Moon happening today / tomorrow, I have deferred a description of the Moon’s seas and craters until Part 3 of this series.

Happy moon observing! Because you’re only going to see the brightest stars right now, skies permitting. If it’s too cloudy, there’s a link on the first reference, the National Geographic article below where you can watch it online.

Have a great day, and Happy Spring (or Fall, if you’re reading this from south of the Equator)!

Previous in series: Observing the Moon, Part 1: Appearance and Phases


Fazekas, Andrew. (2019, March 19). See the first supermoon on the spring equinox in 19 years. Retrieved from

McClure, Bruce, and Deborah Byrd. (2019, January 1). How many supermoons in 2019? Retrieved from

Wikipedia. (2019, March 20). Equinox. Retrieved from

Wikipedia. (2019, March 14). Supermoon. Retrieved from

Wikipedia. (2019, February 22). Syzygy (astronomy). Retrieved from

Zodiac Constellation # 2: Cancer

By Peter Forrester | March 9, 2019

Next in our series of Zodiac Constellations is Cancer, the crab. In case you’re wondering, cancer is Latin for crab, and the disease is named after the crab, rather than the other way around.

Cancer is just to the left of Gemini. It is visible in the early evening in the eastern sky. However, it is the second dimmest of the 12 zodiac constellations. Its brightest star, Beta Cancri, only has an apparent magnitude of 3.5, and there is only one other star that is brighter than 4th magnitude. So you will need a clear moonless sky and be removed from city lights in order to see it. This is the best month to see it, around 9 pm. The Sun is located in Cancer between July 20 and August 9, although astrologers will use other dates for their sign of the zodiac called Cancer.

Just to the left of Cancer is another bright constellation, Leo the Lion. So the best way to find Cancer is to look in between Leo and Gemini. Look for a shape somewhat like an upside-down Y. The points are often identified as the pincers of a crab, but it has been described as various other creatures by ancient societies.

Ten stars in Cancer are known to have planets. One of them, called 55 Cancri, has 5 known planets – one a bit bigger than Earth, and the other 4 being gas giants. One of these gas giants is in the “habitable” zone where, based on its distance from the star, liquid water could exist and therefore life similar to what we have on Earth.

Another prominent object in Cancer is an “open cluster” of stars called Praesepe (Latin for manger), also known as the Beehive Cluster. It is located about 600 light-years from Earth, but is still one of the closest open clusters, defined as “a group of up to a few thousands stars that were formed from the same giant molecular cloud and have roughly the same age”.

Praesepe has an apparent magnitude of 3.7, very similar to Beta Cancri. The name comes from the ancient Greeks and Romans, who saw it as a manger out of which two donkeys were eating. This cluster was one of the first things Galileo Galilei looked at with his telescope in 1609. He saw 40 stars in Praesepe, which just looks like a nebula or cloud to the naked eye (best time to observe it is from February to May). Three planets are now known to exist orbiting two Sun-like stars in the cluster.

Coming back to Earth, the Tropic of Cancer is named for the constellation. It is a line you will see on a globe, the northern edge of a band around the middle of the planet known as the Tropics. It is about 23.5 degrees north of the Equator, and the line is the furthest north where the Sun can be directly overhead. Hawaii is located just south of this line (except for the many uninhabited islands at the northwest end of the island chain). The line is called after the constellation Cancer because the time the Sun is overhead is during the summer solstice (June 20), during which the Sun used to be located in Cancer (it is now in Taurus, because of a process called “precession of the equinoxes” or “axial precession”). The southern edge of the Tropics is the Tropic of Capricorn, which is the same distance south of the Equator.

I wish you all the best in your continued effort to admire these amazing objects in the sky above you. And soon it will be warm enough that you actually want to go out there at night. By the way, don’t forget to turn your clocks ahead tonight or in the morning for our annual tradition called Daylight Savings Time (at least for most of the United States).

Previous in series: Zodiac Constellation #1: Gemini


Wikipedia. (2019, January 11). Axial precession. Retrieved from

Wikipedia. (2019, February 14). Beehive Cluster. Retrieved from

Wikipedia. (2019, February 14). Cancer (constellation). Retrieved from

Wikipedia. (2019, March 6). Open cluster. Retrieved from

Wikipedia. (2019, March 5). Tropic of Cancer. Retrieved from

Skies Over Milton, March Edition

By Peter Forrester | March 1, 2019

Greetings on this fine morning! Don’t forget Daylight Savings starts on Sunday, March 10.

Here are your March skywatching events, courtesy of

Friday, March 1: Moon near Saturn, 2:00 PM, visible from Southwestern US.

Saturday, March 2: Moon near Venus, 6:00 PM (the planet is bright, magnitude -4.1).

Monday, March 4: Moon at apogee (farthest from Earth) at 6:00 AM.

Wednesday, March 6: New Moon at 11:04 AM.

Monday, March 11: Moon near Mars in the evening sky, 1:00 PM.

Thursday, March 14: First Quarter Moon, 6:26 AM.

Tuesday, March 19: Moon at perigee (closest to Earth) at 3:35 PM.

Wednesday, March 20: Vernal Equinox at 6:01 PM. An important event, this is the astronomical start of spring in the Northern Hemisphere, where the Sun crosses into the Northern Celestial Hemisphere. See the Wikipedia links below for more information.

Also on March 20, Full Moon at 9:42 PM.

Thursday, March 28: Last Quarter Moon at 12:10 AM.

Sunday, March 31: Mars 3 degrees from the Pleiades cluster at 3 AM.

Also there are other events I skipped involving the Moon being seen near other objects after the 11th of March. I really can’t exaggerate too much the value I’ve found from the free monthly downloads from If you haven’t been to this site I highly recommend it.

One note on using skymaps – they give all their times in UTC (Coordinated Universal Time), also called Greenwich Mean Time (GMT) in English-speaking countries. This is the time in Greenwich, England. In the US Eastern Time Zone, our time is 5 hours before UTC when we are in Standard Time, and 4 hours before during Daylight Savings. I have been translating the times for you, but if you are looking at their information you will need to be able to convert the time yourself.

With all this, you ought to have another great month of looking at the stars. I wish you all the best, and looking forward to warmer weather with the start of spring this month!

Previous in series: Skies Over Milton, February Edition


Thalassoudis, Kym. (2000-18). Skymaps. Retrieved March 1, 2019 from

Wikipedia. (2019, February 27). Coordinated Universal Time. Retrieved from

Wikipedia. (2019, February 27). Daylight saving time. Retrieved from

Wikipedia. (2019, February 28). Daylight saving time in the United States. Retrieved from

Wikipedia. (2019, February 19). March equinox. Retrieved from

Wikipedia. (2019, February 28). Pleiades. Retrieved from


Circumpolar Constellations: Cassiopeia

By Peter Forrester | February 23, 2019

Today I begin a series on the constellations that are close to the North Star. Constellations that never set from a person’s perspective are called “circumpolar”. They rotate once around Polaris every 24 hours but never go below the horizon. Which constellations are circumpolar depend on your latitude, or distance from the equator, but from northern latitudes between 40 and 50 degrees north, there are five constellations that are definitely circumpolar, with four of them having at least some fairly bright stars.

Milton’s latitude is 43º 24′ (43 degrees 24 minutes) North. If you imagine a circle sitting on the horizon, with the North Star, Polaris, at its center, then everything inside that circle is circumpolar. So I may extend this series out over more than just the first five parts.

I begin with the one called Cassiopeia (one of Ptolemy’s original 48 constellations in the 2nd Century AD), because it is currently high and easily visible in the early evening. The five brightest stars in this constellation form an “asterism” or group of stars with a very recognizable shape, which depending on its direction can be described as a funny “E”, “W”, “M”, or “3”.

If you imagine another circle around Polaris, this one with the Big Dipper on the edge, then Cassiopeia is also on this circle, across from the Big Dipper. In other words, it’s about the same distance away from Polaris as the dipper, but on the other side. So you will always find her in a northerly direction.

Cassiopeia was a vain queen in Greek mythology, who bragged that she and her daughter Andromeda were more beautiful than the Nereids, the daughters of the sea god Nereus. In anger, the god Poseidon punished her kingdom, and she and her husband were persuaded to sacrifice Andromeda to a sea creature Cetus, but she was fortunately rescued by the hero Perseus. Some depictions of Cassiopeia show her chained to a chair of a type once used in torture, which is said to be the gods’ final punishment.

The brightest star in Cassiopeia, Schedar, or α (Alpha) Cassiopeiae, has an apparent magnitude of 2.2. The γ (Gamma)  star sometimes brightens up to 1.6 but is normally around 2.3 in magnitude. Two of the three other stars in the “W” asterism are between 2.3 and 2.7 in magnitude. The dimmest of the five stars in the asterism has a magnitude of 3.3.

The Milky Way (what appears to be a long, thin cloud but is actually the edge of the disk of the galaxy) runs through Cassiopeia. Thus there are many “deep sky” objects such as star clusters and nebulae, and two supernova remnants (including a famous supernova from the year 1572). There are also several galaxies within Cassiopeia. Some of these deep-sky objects can be seen with the naked eye, but generally it takes a telescope to get a good view.

Cassiopeia was one of the first constellations I learned to identify as a teenager because it is so bright and always so close to the North Star. I hope you will take the time to look up and see if you can find it, and happy stargazing!


The Urban Astronomer. (2008, December 17). Circumpolar Stars. Retrieved from

Wikipedia. (2019, February 12). Cassiopeia (constellation). Retrieved from on February 19, 2019.

Wikipedia. (2019, January 22). Cassiopeia of Ethiopia. Retrieved from on February 19, 2019.

Wikipedia. (2018, January 28). Circumpolar constellation. Retrieved from on February 19, 2019.

Wikipedia. (2019, February 11). Latitude. Retrieved from on February 19, 2019

Bright Stars: Sirius

By Peter Forrester | February 13, 2019

The brightest star in the night sky, as seen from Earth, is called Sirius. It is the second brightest overall (obviously the Sun is the brightest star we can see).

Sirius is larger than the Sun, but weak and relatively dim compared to many other stars, but it is so close to the Sun (8.6 light years) that its apparent magnitude (-1.46) is almost twice as bright as any other star. It has an absolute magnitude of 1.42.

Being so bright, Sirius was important in the mythology of many ancient cultures. For example, its appearance in the sky shortly before sunrise (its “heliacal rising”) occurred just before the summer flooding in ancient Egypt and was an important omen in their religion. Their civil calendar was originally based on the return of Sirius, but wandered off within 4 years. However, they continued to observe its return and discovered a cycle that became part of the Julian and Alexandrian calendars.

The Ancient Greeks associated Sirius with hot weather, men becoming weak, and other bad consequences, terming the time right after its return “dog days” because of its constellation, Canis Major. Its association with dogs also concerned the way dogs pant in hot weather, which made some ancient people fear they might have a disease, even in some cases rabies. For more on the mythologies associated with Sirius, see the Wikipedia link below.

Sirius is in Canis Major, or the Great Dog, which is just to the left of Orion as seen in the Northern Hemisphere. Being the brightest star in this constellation, its abbreviation is α CMa. Canis Major is often depicted as a dog of Orion who is helping him hunt the Bull, Taurus.

Sirius is actually made up of two separate stars, Sirius A and Sirius B. Sirius A has an absolute magnitude of +1.42, while Sirius B (nicknamed “the Pup”) is much, much dimmer at +11.18. The distance between them is about 20 AU, or about the distance of Uranus from the Sun. 1 Astronomical Unit (AU), the distance between the Earth and the Sun, is about 93 million miles. Sirius A is blue-white, while Sirius B is a white dwarf, the second one ever discovered.

The existence of a second star was first hypothesized in 1844, and observed in 1862. A third companion star has been suggested to explain small changes in the movement of the star system but its existence has never been verified.

Sirius is one of the three stars in a shape called the “Winter Triangle”, along with Procyon in Canis Minor, and Betelgeuse in Orion. Those who know the constellation Orion should be able to find Sirius easily. You just follow the belt down to the left, the star is about 8 times the width of the belt away from it. These directions will be opposite in the Southern Hemisphere.

There are four planets brighter than Sirius (two of them, Jupiter and Venus, are always brighter when visible, with Mercury and Mars being brighter some of the time). Since it is known to be the brightest night-time star, knowing its location can help you identify planets in the sky.

It is possible to see Sirius in daylight, though there are several conditions that have to be met, such as the sky being clear, the Sun being low on the horizon, Sirius being overhead, and the observer being at a high altitude. These conditions are met more easily in the Southern Hemisphere, since the star gets higher in the sky there. Sirius is gradually moving to the south and will no longer be visible in northern or central Europe in about 7,000 years.

Sirius contains 2 of the 8 nearest stars to our solar system, and is the fifth closest stellar system to us. The Voyager 2 spacecraft launched in 1977 is expected to pass about 4 light years from Sirius in about 300,000 years. No planets have been detected in the Sirius system, and because of differences in the stars, the conditions for life being met would be much more difficult. In particular, Sirius B used to be a red giant and thus would have swallowed up any planets. Also Sirius A is much brighter than the Sun and thus habitable planets would have to be much further away than the Earth is from the Sun. Sirius A is also much younger than the Sun and thus there would have been much less time for life to have evolved on any planet there.

There are a couple of historical controversies concerning Sirius. The first is about the color of the star, which ancient authors described as red, but the star is now known to be white to blue-white in color (various colors can be seen in its twinkling, because of effects of the light passing through Earth’s atmosphere).

The second controversy concerns a native tribe in Mali, Africa, which has been claimed to have known about the second star before Western astronomers (they also said there was a third star with a planet). But the accounts have been disputed and it seems the author who wrote about it in 1938 may have been the one to tell them, or they may have learned about it from a French expedition to view an eclipse in 1893.

This is the best month of the year to observe Sirius in the early evening, although it can also be seen in the early morning during summer. Happy skywatching!


Byrd, Deborah. (2019, February 7). Sirius is Dog Star and brightest star. Retrieved from on February 13, 2019.

Howell, Elizabeth. (2018, October 24). Sirius: Brightest Star in Earth’s Night Sky. Retrieved from on February 13, 2019.

Wikipedia. (2018, December 31). Heliacal rising. Retrieved from

Wikipedia. (2019, February 12). Sirius. Retrieved from