Leo, the Lion
Dancing with Mars, plus the planets for May.
High in the southern sky on May evenings is the constellation Leo, the Lion. This is one of the oldest constellations in the sky. Its history can be traced back to the Mesopotamians around 4000 BCE.
More recent mythology from the Greeks associates this constellation with the Twelve Labors of Hercules, whose first labor was to kill the Nemean Lion. The lion's golden fur was impermeable to weapons and it had been kidnapping women from surrounding towns and chaining them up in its cave. When would-be heroes would show up to rescue a chained woman, the apparent woman would be transformed into the lion and consume the hero, who could not penetrate its fur. Hercules managed to trap it in its cave and strangle it using his tremendous strength. He would later use the lion's pelt to protect himself during subsequent labors.
Right now, Leo is host to the planet Mars. Mars has been in or very close to Leo since mid-October last year and will remain there until mid-June this year. Usually, a planet like Mars will zip through a constellation in a month, but Mars reached opposition in March, so it did a little dance in Leo.
Planets move eastward in their orbits around the Sun (counter-clockwise as viewed from north of the Sun), so they usually move eastward in our sky as well. This is called direct motion. Mars was moving eastward in our sky last October when it entered Leo. The Earth was on the back side of the Sun, swinging around to the Mars-facing side.
Earth moves more rapidly in its orbit than Mars, since it is closer to the Sun. As we swing around the Sun, the faster motion of the Earth makes Mars appear to move westward in our sky, even though it is moving eastward in its orbit. Think of two runners racing on a circular track, with the faster runner in the inner lane. From the stands, we see the two runners running eastward. The faster runner sees the slower one on the outside track moving eastward, but as Mr. Fast approaches Mr. Slower, Mr. Fast sees Mr. Slower appear to be moving backward (westward). The same thing happens with Earth (Mr. Fast) and Mars (Mr. Slower).
Last October, when Mars entered Leo, Earth's faster speed was mostly used to swing around the Sun. As it started moving more parallel to Mars, Earth's greater eastward motion overwhelmed Mars' slower pace and it first appeared to stop, and then move backward (westward). This is called retrograde motion. The stopping point is called the stationary point and occurred on Jan. 25, just over the border from Leo in Virgo. With the Earth and Mars running parallel, Mars now appeared to move westward back into Leo. The Earth passed Mars at opposition on March 3.
Mars continues to move westward in our sky until the Earth curves away from Mars, following its orbit around the Sun. Earth's greater speed is now used to swing away from Mars, so it no longer overwhelms Mars' speed. Mars stopped once again at another stationary point, under the belly of Leo, on April 15. Mars now resumes direct (eastward) motion and finally exits Leo on June 20. This dance will not occur again until we get near the next opposition in April 2014.
All of the outer planets do this dance. The farther from the Earth and Sun the outer planet orbits, the smaller the loops are in our sky. The inner planets (Mercury and Venus) go faster than the Earth, so they do not go through this kind of retrograde loop, but that is a story for another day.
The Planets for May 2012
Having been passed by Venus in March, Jupiter is now too close to the Sun to be observed. The Goddess of Love is still up in the western sky as it starts to get dark, but not for much longer. Venus will be one degree south of the second-magnitude star Elnath on May 7. That's almost the limit of Venus' eastward travels, though; it will have turned around by May 15, accelerating back toward the Sun. By the last week of the month Venus will be lost in the Sun's glare. Venus spends the entire month in eastern Taurus. On May 1, Venus will be glowing at magnitude -4.5 with a disc that is a 26% illuminated crescent and 38.0 seconds-of-arc across.
Watch the Skies
Mars is in southern Leo, moving slowly eastward. It sets around 2:30 a.m. At midmonth, the God of War's disc is 8.6 seconds-of-arc across and it is magnitude -0.3. Mars is 66 degrees up in the south as it gets dark.
Saturn is a little farther east in Virgo, moving slowly westward among the stars. Just past opposition, Saturn is 40 degrees up in the southeast as it gets dark and sets around 5 a.m. At midmonth, the Ringed Planet's disc is 18.7 seconds-of-arc across while the Rings are 42.4 seconds-of-arc across, tipped down 12.8 degrees showing their northern face.
Mercury is in the morning sky for the first two-thirds of the month, but the ecliptic is tilted down toward the horizon so Mercury is not very high in the sky. The Messenger of the Gods made its morning appearance last month and is already heading back toward the Sun. It will be gone by midmonth. On May 1, it rises at 5:30 a.m. and shines at magnitude -0.1. The disc is 65% illuminated and 6.4 seconds-of-arc across. Mercury moves eastward from Pisces, through Aries and into Taurus during the month.
During the late afternoon of May 20, the Sun, Moon and desert southwest will line up, producing an annular solar eclipse. The Moon is farther from the Earth than average, so it will appear smaller than the Sun. When it tries to cover the Sun, the Moon will be unable to cover the Sun's whole disc, so the sun will appear as a bright ring of fire in the sky. But you have to be in the right place to see the ring. Most areas will see the Moon cover only part of the Sun, but if you are in the path of annularity, a 190-mile-wide west-northwest to east-southeast swath, you will see the ring. The path starts in eastern China, crosses southern Japan, traverses the Pacific Ocean, and enters the United States in northern California. It continues across southern Nevada, the Utah-Arizona border and into north-central New Mexico, ending at sunset near Lubbock, Texas.
For us, the center of the path goes through Tohatchi, NM (just north of Gallup), southern Albuquerque and midway between Clovis and Roswell. The Sun will be very low, only about five degrees above the western horizon, so you will need a good western horizon. The Ring of Fire will last about four and a half minutes. If you are within the path of annularity, but not near the centerline, you will see the Moon off-center but will still see the ring, though for a shorter time.
If you view the eclipse, you will need a solar filter. It will be just like looking at the uneclipsed Sun, so be very careful not to look at it directly or you could damage your eyes. Further information on this event is on the Internet. So enjoy this exciting event and "keep watching the sky"!
An amateur astronomer for more than 40 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.