Skies over Milton, May Edition

By Peter Forrester | May 9, 2019

Greetings, stargazers, wherever you may be! Apologies for the long delay in writing this update – I have been dealing with some personal issues which took up a great deal of my time every day, for about the last month, but that has now been resolved.

So here are your skywatching tips for this “Merry Month of May”!

Past events (included only for completeness, except for the meteor shower which is ongoing):

Thursday, May 2: Moon near Venus, morning sky at 11:00 AM (Eastern Daylight Time)

Friday, May 3: Moon near Mercury, morning sky at 6:00 AM.

Saturday, May 4: New Moon at 6:46 PM.

Monday, May 6: Three events: Moon near Pleiades (open cluster) at 1:00 AM.

Also, the peak of the Eta Aquariid meteor shower occurred at 10:00 am (most visible for a 7-day period around this peak, still visible now just above the horizon before dawn). This meteor shower is formed by debris that separated from Halley’s Comet hundreds of years ago. Easier to see in the Tropics and in the Southern Hemisphere. The shower lasts until Tuesday, the 28th of May.

Third, the Moon was near the bright star Aldebaran at 6 PM.

Tuesday, May 7: The Moon was near Mars at 9:00 PM.

Future events: 

Thursday, May 9: The Moon will be near the bright star Pollux (one of the bright ones in Gemini) at 11:00 PM.

Saturday, May 11: First Quarter Moon at 9:12 PM.

Monday, May 13: Moon will be at perigee (closest to Earth in the current orbital period) at 5:54 PM.

Saturday, May 18: Full Moon at 5:10 PM.

Tuesday, May 21: Mercury will be at superior conjunction (meaning it is directly behind the Sun) with the Sun at 9:00 AM. After this it will pass into the evening sky.

Wednesday, May 22: Moon will be near Saturn at 4:00 PM. People in South Africa will be able to see an “occultation”, meaning that Saturn will be behind the Moon.

Sunday, May 26: Moon at apogee (furthest from the Earth) at 9:00 AM. Also, Last Quarter Moon at 12:33 PM.

Tuesday, May 28: Ceres will be at opposition at 6:00 PM. This means that it is in a straight line with the Earth and Sun, with the Earth in the middle. This is when it is brightest, but at apparent magnitude of 6.7, it is still too dim to see with the naked eye unless you have extremely dark skies.

Ceres is the largest object in the Main Asteroid belt between Mars and Jupiter, and the only “dwarf planet” there. It was the first asteroid to be discovered, way back in 1801. The NASA spacecraft Dawn orbited Ceres from 2015 to 2018, see the reference below for more information on this fascinating space body.

For more skywatching events (including some in the free download PDF that involve the Moon being near various objects which I omitted), or to see the events in Universal Time or ones that are not visible from my location in Milton, New Hampshire, see http://Skymaps.com/skycalendar/.


Previous in series: Skies Over Milton, April Edition


References:

Thalassoudis, Kym. (2000-19). Skymaps. Retrieved May 9, 2019 from skymaps.com.

Wikipedia. (2019, May 8). Ceres (dwarf planet). Retrieved from https://en.wikipedia.org/wiki/Ceres_(dwarf_planet).

Wikipedia. (2019, April 20). Conjunction (astronomy)#Superior and inferior. Retrieved from https://en.wikipedia.org/wiki/Conjunction_(astronomy)#Superior_and_inferior

Wikipedia. (2019, May 4). Eta Aquariids. Retrieved from https://en.wikipedia.org/wiki/Eta_Aquariids.

Wikipedia. (2019, May 8). Opposition (astronomy). Retrieved from https://en.wikipedia.org/wiki/Opposition_(astronomy).

Circumpolar Constellations: Ursa Major

By Peter Forrester | April 18, 2019

Greetings all! Let’s talk about one of the best-known group of stars (not a constellation) in the sky.

Yes, I mean the Big Dipper, which is referred to as an “asterism,” or a collection or shape of stars that are not recognized as a constellation (the 88 “official” ones were defined in 1928, at a meeting of the International Astronomical Union, which just celebrated its 100-year anniversary a few days ago).

Officially, by the way, a constellation is a region in the sky, not just a set of bright stars that form a shape but everything around them as well. For instance, when the Sun, Moon, or a planet is in the same direction as a constellation we say that it is “in” that constellation. The same is true of comets, asteroids, nebulas, quasars, pulsars, black holes, whole galaxies, or even dust. Every point in the sky is defined as belonging to one or more constellations (the “more” being along the borders).

But back to the asterism known as the Big Dipper. In this case, the 7 stars that make up the Dipper (called the Plough in the U.K.) are part of the constellation Ursa Major (the Great Bear).

I have not covered the Big Dipper until now because as of a few months ago, it was still very low to the horizon in the early evening, and thus not very visible if you have any kind of treeline.

Six of the seven stars in the Dipper are of 2nd degree magnitude, and the other one is 3rd degree. They are the brightest stars in Ursa Major.

Many of my readers will already know that the Big Dipper, as I said about Orion, can be used to find other stars and constellations easily. Tracing the side of the bowl upward, the line extends to Polaris, the North Star, which is part of the constellation called Ursa Minor. This is very useful for nighttime navigation without a compass, as Polaris is always located to the north. Sailors have been using the stars to find their direction for millennia, and the Big Dipper was what they used to locate Polaris, since Polaris is only 3rd degree and looks similar to every other star of the same color and brightness.

Using the same two stars that form a line to Polaris, tracing the line in the opposite direction brings you to another bright constellation, one of the Zodiac: Leo the Lion, whose shape resembles a rectangle that touches a backwards question mark. Thirdly, following the curve of the handle brings you to the constellations Bootes (resembling a kite, its brightest star called Arcturus) and Virgo (brightest star Spica). Hence the once-popular saying “Arc to Arcturus, and spike to Spica”). There are some other lesser known direction-finding lines using different stars in the Big Dipper, see the Big Dipper reference below if you want to know more about this.

Ursa Major is very close to the north celestial pole, and thus as explained previously, it  is circumpolar. Therefore, meaning it never sets from the sky when seen from northern latitudes such as the United States or Europe, but appears to go in a circle every 24 hours around Polaris, the star located directly over the North Pole of Earth.

Incidentally, the Big Dipper has been considered as the tail of a bear, and various other images (this one makes no sense because bears don’t have long tails). The rest of the bear consists of dimmer stars to the right of and below the bowl when it is lying flat on its bottom (the bowl to the right of the handle – it is close to the horizon when it is in this direction).

In popular culture, the Big Dipper is found on the flag of the state of Alaska, and a few other flags and coats of arms. It may even be mentioned in the Bible (its reference in the book of Job is disputed by scholars).

I once made a scale-model view of the distance of the stars in the Big Dipper, a side-view, if you will. One of the stars was so much further than the rest that I had to mount it on a separate piece of cardboard from the rest. This was a school project, and unfortunately I no longer have it. But it is interesting to note that the shapes we see are only from our perspective, and most constellations would appear very different in other solar systems.

I’m going to leave the discussion there, because I’m feeling a bit under the weather, but I wish you all the best time while skywatching, and be sure to look up when you go out!


Previous in series: Circumpolar Constellations: Cassiopeia


References:

Wikipedia. (2019, March 10). Asterism (astronomy). Retrieved from https://en.wikipedia.org/wiki/Asterism_(astronomy).

Wikipedia. (2019, April 2). Big Dipper. Retrieved from https://en.wikipedia.org/wiki/Big_Dipper.

Wikipedia. (2019, April 1). Constellation. Retrieved from https://en.wikipedia.org/wiki/Constellation.

Wikipedia. (2019, March 29). Ursa Major. Retrieved from https://en.wikipedia.org/wiki/Ursa_Major.

Skies Over Milton, April Edition

By Peter Forrester | April 2, 2019

Greetings on this fine sunny day! (I wish I could call it “warm”!)

Here are your monthly skywatching events, including one that already occurred. Note: all times are in US Eastern Daylight Savings Time (EDT) unless otherwise noted, which is 4 hours less than Universal Time (UT). Note: not all of these events occur at a time where the sky in Milton will be dark enough to see them.

Tuesday, April 2: Moon near Venus, 3:00 am EDT. Also Moon near Mercury, 10:00 PM.

Friday, April 5: New Moon at 4:51 AM. This is the start of “Lunation 1191” (a lunation is about 29.53 days, or the time between one new moon and the next. The exact length of a lunation varies a little bit, see the reference on “New moon” for more information).

Monday, April 8: Moon near the Pleiades at 7 pm.

Tuesday, April 9: Moon near Mars at 6 am (evening sky). Moon near Aldebaran (brightest star in constellation Taurus) at noon (in evening sky).

Friday, April 12: First Quarter Moon at 3:05 PM.

Sunday, April 14: Mars near Aldebaran at 9 pm.

Friday, April 19: Full Moon at 7:11 AM.

Monday, April 22, Lyrid Meteor Shower peaks at 8 pm. It is visible from the 14th to the 30th. 10 – 20 fast, bright meteors per hour during the peak, unfortunately the Moon’s brightness well make these meteors harder to see. The meteors can be seen all over the sky, but the radiant (point of origin) is between the constellations Lyra and Hercules.

Tuesday, April 23: Moon near Jupiter, 9:00 AM.

Thursday, April 25: Moon near Saturn at 9:00 AM (occultation, which means the nearer object passing in front of the other and therefore blocking light from it, will occur. In this case, the occultation will only be visible in parts of Australia, New Zealand, and South America).

Friday, April 26: Last Quarter Moon at 6:18 PM.

For more of these events, or to check out other skywatching tips, be sure to visit skymaps.com.

Until my next, enjoy the skies, and this glorious spring which is now upon us (at least in the Northern Hemisphere)!


Previous in series: Skies Over Milton, March Edition


References:

Thalassoudis, Kym. (2000-19). Skymaps. Retrieved April 2, 2019 from skymaps.com.

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

Wikipedia. (2019, January 10). New moon. Retrieved from https://en.wikipedia.org/wiki/New_moon.

Wikipedia. (2019, March 10). Occultation. Retrieved from https://en.wikipedia.org/wiki/Occultation.

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!


References:

Wikipedia. (2019, March 15).  Comet Shoemaker-Levy 9. Retrieved from https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9 .

Wikipedia. (2019, March 13). Galileo (spacecraft). Retrieved from https://en.wikipedia.org/wiki/Galileo_(spacecraft).

Wikipedia. (2019, March 18). Galileo Galilei. Retrieved from https://en.wikipedia.org/wiki/Galileo_Galilei.

Wikipedia. (2019, March 22). Juno (spacecraft). Retrieved from https://en.wikipedia.org/wiki/Juno_(spacecraft).

Wikipedia. (2019, March 26). Jupiter. Retrieved from https://en.wikipedia.org/wiki/Jupiter.

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 timeanddate.com (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


References:

Time and Date A.S. (1995-2019). Night Sky Map & Planets Visible Tonight. Retrieved from https://www.timeanddate.com/astronomy/night/.

Wikipedia. (2019, March 26). Jupiter. Retrieved from https://en.wikipedia.org/wiki/Jupiter.

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


References:

Fazekas, Andrew. (2019, March 19). See the first supermoon on the spring equinox in 19 years. Retrieved from https://www.nationalgeographic.com/science/2019/03/see-first-super-worm-moon-on-spring-equinox-in-19-years/.

McClure, Bruce, and Deborah Byrd. (2019, January 1). How many supermoons in 2019? Retrieved from https://earthsky.org/human-world/what-is-a-supermoon.

Wikipedia. (2019, March 20). Equinox. Retrieved from https://en.wikipedia.org/wiki/Equinox.

Wikipedia. (2019, March 14). Supermoon. Retrieved from https://en.wikipedia.org/wiki/Supermoon.

Wikipedia. (2019, February 22). Syzygy (astronomy). Retrieved from https://en.wikipedia.org/wiki/Syzygy_(astronomy).

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


References:

Wikipedia. (2019, January 11). Axial precession. Retrieved from https://en.wikipedia.org/wiki/Axial_precession.

Wikipedia. (2019, February 14). Beehive Cluster. Retrieved from https://en.wikipedia.org/wiki/Beehive_Cluster.

Wikipedia. (2019, February 14). Cancer (constellation). Retrieved from https://en.wikipedia.org/wiki/Cancer_(constellation).

Wikipedia. (2019, March 6). Open cluster. Retrieved from https://en.wikipedia.org/wiki/Open_cluster.

Wikipedia. (2019, March 5). Tropic of Cancer. Retrieved from https://en.wikipedia.org/wiki/Tropic_of_Cancer.