April 22, 2015
Find your way around the night sky! Below is a free sky map (star chart) as well as a printable version, courtesy of astronomer Jeff DeTray.
Sky Map for May 2015
Each month, Jeff’s Sky Maps provides a sky map which highlights beautiful events in the evening sky—stars, constellations, planets, conjunctions with the Moon, meteor showers, and other amazing celestial objects. Follow more of Jeff’s sky adventures at AstronomyBoy.com.
Click-and-Print Sky Map
Just click here or on the image below to open the printable map—then bring outside!
Sky Map Highlights:
The Giant Planets and the Lion
There are two types of planets in our solar system.
The four “rocky planets” are closest to the Sun and include Mercury, Venus, Earth, and Mars. These planets are relatively small and have solid surfaces containing significant amounts of rock and metals. They are also known as the “terrestrial planets” because they resemble Earth in their size and composition.
Farther from the Sun are Jupiter, Saturn, Uranus, and Neptune, the “giant plants.” They are much, much larger than the rocky planets and contain less solid material and more material in the form of gases, liquids, and “ices” (frozen liquids).
Uranus and Neptune, the farthest of the giants, are both comprised of a small rocky core surrounded by a layer of frozen water, frozen ammonia, and frozen methane under a layer of gases that include hydrogen, helium, and methane. Both of these planets are quite large compared to Earth: Uranus is 14 times and Neptune is 17 times the mass of Earth. Neither Uranus nor Neptune has a solid surface, so you could not walk on either one. Under pristine skies and with excellent eyesight, it is just possible to see Uranus without optical aid, but it is not currently visible in the nighttime sky. Neptune always requires binoculars and telescopes to be seen.
Closer, larger, and brighter than Uranus and Neptune and therefore very easy to see are the two “gas giant” planets, Jupiter and Saturn. Both are shown on this month’s Sky Map. Go outside in the late evening to see them. Both are composed mainly of gases, with lesser amounts of ices. As with the other giant planets, neither Jupiter nor Saturn has a solid surface. If space explorers ever visit them, landing will be impossible. A space craft could orbit the planets or set down on a moon, but no one will ever set foot on Jupiter or Saturn.
Jupiter is the king of the planets, the largest in our solar system, and more than 300 times as massive Earth. In fact, Jupiter contains two-and-a-half times the mass of all the other planets in our solar system combined. Jupiter is composed almost entirely of gases. Look to the southwest, and find the brightest object in the sky. That’s Jupiter. The planet will be visible until about 2:00 a.m., after which it will set below the horizon.
Sky map produced using Chris Marriott’s Skymap Pro
Saturn is smaller than Jupiter but it also dwarfs Earth, with almost 100 times our planet’s mass. Its composition is similar to that of Jupiter. During May, Saturn is brighter than it has been for several years, and it will be visible all night long during the month.
Each night during May, Saturn can be spotted in the late evening, low in the southeast. Through the course of the night, it will appear to move gradually toward the south and by early morning will be low in the southwest. We say Saturn “appears to move” but in reality this apparent motion during any one night is caused entirely by the Earth rotating on its axis. All of the stars and planets appear to move in this way, travelling together from east to west through the sky each night. This nightly parade is due solely to the rotation of Earth. You can think of the sky as stationary, with Earth eternally rotating beneath it.
The planets, including Earth, have some independent motion, but it can take weeks or months for that motion to be noticeable. For example, if you carefully note the location of Saturn with respect to a few nearby stars (make a simple sketch of their relative positions) and then check again a month later, you will see that Saturn has moved a small amount with respect to the stars.
The other planets also move with respect to the stars, some faster and some slower than Saturn. In fact, the word “planet” comes from the Greek expression for “wandering star.” Ancient astronomers noticed that while the positions of most stars never change with respect to one another, a very few of them seemed to wander. This handful of “wanderers” came to be known as planets.
To summarize, the nightly movement of the stars and planets across the sky is due to the rotation of Earth. The long-term changes in the positions of the planets with respect to the stars is due to the independent motion of the planets. Depending on the planet, this may take weeks or months to be noticeable.
The Constellation, Leo
The month of May is an excellent time of year to view the large constellation Leo, the Lion. As you can see on the map, the outline of Leo resembles its namesake, with its head, tail, and legs all comprised of relatively bright stars. The very bright star Regulus lies at Leo’s heart. In fact, the Arabic name for the star means “heart of the lion.” It should come as no surprise that Denebola, the star at the other end of Leo, derives its name from the Arabic phrase for “tail of the lion.” Leo is larger than you might think looking at the map, so think big as you look for the celestial Lion. Start by finding Jupiter, then look to Jupiter’s left to find Regulus, the only other bright object anywhere near Jupiter.
See our Sky Watch page for more highlights of the monthly sky, courtesy of The Old Farmer’s Almanac.