If the planets are your favorite objects to view through a telescope or even just by eye, then May has a show in store. Mercury, Venus, Mars, Jupiter, and Saturn are all bright enough to be seen with the unaided eye and thus have been known since antiquity. Except for Mars, all of these planets will put on an evening performance for skywatchers in May.
By far the hardest to spot of the four planets will be Mercury. Mercury’s small orbital radius (about one-third the Earth-Sun distance) never allows the planet to stray far from the Sun, so most of the time it is easily lost in the glow of sunrise or sunset; however, Mercury reaches greatest eastern elongation on May 7, which simply means that it will appear about as far from the Sun in the evening sky as it can get. During this time Mercury can be found as a moderately bright “star” just above the west-northwestern horizon after sunset, and the planet will take on a very small, crescent-moon shape when viewed through backyard telescopes. Mercury and Venus both go through a complete cycle of phases for the same reason that our Moon exhibits phases. At all times, half of the surfaces of Mercury, Venus, the Moon, and any other round body orbiting the Sun are always illuminated just as half of the Earth is experiencing daytime as you read this. Using his small telescope Galileo Galilei was the first to observe this in the early 1600s, and these observations provided direct observational proof that some objects, namely Venus, go around the Sun and not the Earth. In addition, as these planets move closer to and farther away from us as we all orbit the Sun, they appear to enlarge and shrink in telescopic views due to the changing distances between us and them. If you get a chance to observe Venus and Mercury through a backyard telescope, you will see both of these phenomena quite well.
Speaking of Venus, even casual observers will have noted the bright planet in the western sky after sunset. Over the past few months, Venus has been creeping higher up in the evening sky as it rounds the Sun to catch up to the Earth. For the next few months, Venus will only be outshone by the Moon in the evening sky as it continues parading as the “Evening Star.” Venus’s high brightness is due to its clouds, which are highly reflective and completely enshroud the Venusian surface. Just like Mercury, Venus’ orbit will never let it stray far from the Sun in our skies, but its larger orbital radius lets it get significantly higher in the sky as compared to Mercury. On June 6, Venus will reach greatest eastern elongation and show a distinctive half-moon phase when viewed telescopically. From this point onward, it will grow larger in telescopes as it takes on a thinner and thinner crescent phase. When it finally catches up to and passes us in August, Venus will begin creeping up in the eastern sky before sunrise as it changes to its “Morning Star” persona.
Jupiter continues to ride high in the sky just after sunset. The most massive of the solar system’s planets puts on a new display each night as its four largest moons, also discovered by Galileo, continually change positions. Folks with medium- to large-aperture telescopes will note that each of these moons appears as a small disk under high magnification as opposed to pinpoints of light. Though these moons are roughly a half-billion miles from us, they are large enough (about the size of our moon, to first order) that even backyard telescopes can resolve them. Occasionally we are treated to additional performances by the moons as their ink-black shadows are cast on the planet and sweep across its atmosphere in a matter of hours.
Finally, the real gem of the solar system makes its debut in the evening skies by early May. During the first part of May, Saturn will appear as a bright “star” near the east-southeastern horizon just above the star Antares, the brightest star of the constellation Scorpius. As the month progresses, Saturn will gradually slide west among the stars and move from Scorpius to Libra. Normally, as Earth and the other planets orbit the Sun, one would expect the planets to move eastward among the stars. However, as Saturn approaches opposition on May 23 (another fancy term that simply means Saturn rises as the Sun sets), it will appear to move backward in its orbit as Earth catches up to and passes it. This retrograde motion will continue until the start of August at which point skywatchers will note that Saturn will halt its backwards movement and resume its normal, easterly trek among the stars. Opposition is a great time to view Saturn as it means that the planet will also be visible for the majority of the night, giving those with telescopes plenty of time to admire the giant rings of the planet and even spot a few of its moons.
During the start of May, around 10pm CDT, folks with clear eastern and western horizons can spot all four of these planets simultaneously. Starting low on the west-northwestern horizon one can first spot dim Mercury, then brilliant Venus high up in the western sky, then Jupiter almost overhead, and finally Saturn near the east-southeastern horizon. If the ground were transparent and no atmosphere was present, one could continue this line, known as the ecliptic, around to form a complete circle and also intercept the planets Uranus, Neptune, and Mars. The Moon would also lie fairly close to this line, and the Sun would lie directly on top of it. This alignment is not coincidence – the ecliptic is the orbital plane of the Earth, or as viewed from Earth, it marks the apparent path of the Sun through our sky. The planets and Moon do not stray far from the ecliptic because the orbital planes of Earth and the planets are in close alignment, and this alignment is a result of the formation of the solar system. Approximately five billion years ago, a large, rotating cloud of gas and dust began collapsing in on itself due to its own self-gravity. As the rotating cloud collapsed to a smaller, more compact size its rotation speed increased just as a rotating ice skater spins faster as she pulls her arms closer to her body. But, as anyone who has spun around has experienced, centrifugal force causes a spinning ice skater’s arms to want to fly outwards – this is also experienced by the collapsing cloud. As the cloud collapses down, it not only spins faster but centrifugal force tries to halt the collapse in the direction perpendicular to the cloud’s rotation axis but not the collapse along the rotation axis. The end result is the cloud flattens down to become a rotating disk – the Sun forms at the center of the disk and planets form in the outer portion. The planets slowly build up in the rotating disk of material and, once the cloud of material is depleted/dissipated, the newborn planets continue their orbits around the Sun in approximately the same orbital plane.