Crater, the Cup
Plus the planets for May.
Just a little bit east of south on these May evenings you can find the constellation Crater, the Cup, slightly less than halfway up in our evening sky. Crater's dim stars represent a cup or goblet belonging to the god Apollo. It is 53rd in size out of the 88 officially recognized constellations. Its brightest stars are only fourth magnitude, making it difficult to find from a light-polluted observing location.
The mythology surrounding this constellation is actually focused on the neighboring constellation, Corvus, the Crow. It seems that Apollo got thirsty one day and gave his cup to Corvus, instructing him to fetch some spring water. Corvus promptly flew down to Earth, but as he headed toward the spring, he spied a fig tree with big, almost-ripe figs. Forgetting his task, he headed for the fig tree and spent a couple of days waiting for the figs to ripen. After gorging himself on the figs, he remembered his now-much-delayed task and he headed toward the spring to fill the cup. Realizing that Apollo would be angry with him, he grabbed a nearby water snake and took it with him back to Olympus.
When he arrived there, Corvus told Apollo that the water snake had attacked him and that is why he was late. Apollo knew he was lying, and angrily threw the cup (Crater), the water snake (Hydra) and the crow (Corvus) into the sky to become members of our modern constellations. A slightly different version has Apollo putting Corvus on the back of Hydra, the threatening multi-headed water snake that Hercules conquered as one of his labors. In either case, the cup is there to remind Corvus of his misdeed for all eternity.
There are not many interesting objects in Crater, but there are a few galaxies. One interesting pair is NGC 3511 and NGC 3513. These two spiral galaxies are only 11 minutes-of-arc apart and they fit in the same telescopic view. These two are also about the same brightness.
NGC 3511 is the northeast member of the pair, shining with a total magnitude of 11.6. It is 45 million light-years from us. This galaxy has a mottled appearance and is tilted 45 degrees to our line of sight. NGC 3511 is 5.7 by 2.0 minutes-of-arc in size, making it rather elongated. It has a slight central bar, making it a type SBc galaxy. William Herschel discovered it on Dec. 21, 1786.
Its neighbor, NGC 3513, is also a barred spiral of type SBc. It has a strong central bar with an arm swinging clockwise from each end of the bar. We are seeing it face-on, so it appears more circular, 2.8 by 2.3 minutes-of-arc in size. It is magnitude 12.1.
You notice they are both type SBc as classified in Edwin Hubble's "Tuning Fork" galaxy classification system, invented in 1926. Hubble divided the galaxies into three large classes: ellipticals, spirals and barred spirals. A fourth class, the irregulars, was not included on the diagram, because he thought that his diagram represented the actual evolution of galaxies. We know today that it does not.
The Hubble Sequence is arranged like a tuning fork on its side. The base of the tuning fork is on the left, starting with elliptical galaxies that are spherical in shape (type E0). As you move along the base of the tuning fork, the galaxies become more elliptical (E1 to E9). At the junction of the base and arms are the lenticular galaxies (S0). They are shaped somewhat like a spiral galaxy, but have no arms, just a smooth disc.
Continuing along the arms of the tuning fork, the upper arm is the regular spiral galaxies. Type Sa has tightly wound arms; Sb is a little looser; Sc is very loose, with star clusters and nebulae plainly visible. The lower tuning fork arm represents the barred spiral galaxies, classified in the same way as the regular spirals, from type SBa to SBc like NGC 3511.
In 1959, French astronomer Gérard de Vaucouleurs, working at Harvard Observatory, proposed a change to Hubble's Tuning Fork, adding (among other things) types Sd and SBd, which have very loose or fragmentary arms with most of the luminosity of the galaxy in the arms and not in the central bulge. The classification of galaxies continues to evolve, but the Hubble Tuning Fork is still the most commonly used system.
The Planets for May
Mars is still too close to the Sun to be seen this month. Venus and Mercury, however, are moving away from the Sun and back into the evening sky. Venus, Mercury and Jupiter will all be close together on March 26 when they will be within two and a half degrees of each other.
Watch the Skies
May 2, 5:14 a.m. — Last Quarter Moon
May 5, midnight — Eta Aquarid Meteor Shower peaks
May 9, 6:28 p.m. — New Moon/Annular Eclipse
May 17, 10:34 p.m. — First Quarter Moon
May 24, 9 p.m. — Mercury 1.4° N of Venus
10:25 p.m. — Full Moon/Penumbral Lunar Eclipse
May 27, 3 a.m. — Mercury 2.4° N of Jupiter
May 28, 2 p.m. — Venus 1.2° N of Jupiter
May 31, 12:58 p.m. — Last Quarter Moon
As we get into the last third of the month, you will find Venus and Mercury low in the west-northwest as it starts to get dark. They are both coming out from behind the far side of the Sun. Mercury starts the month in Aries. Moving eastward, it quickly enters Taurus, traversing the entire constellation and popping over the border into Gemini as the month ends. At that time, Mercury will be magnitude -0.3 with a 63% sunlit disc that is 6.5 seconds-of-arc across. Mercury sets at 9:45 p.m.
Venus is also traveling eastward, but more slowly. It starts the month in Aries and moves into Taurus, where it ends the month in the eastern part of that constellation. Venus gets passed by the faster Mercury on March 23. By the end of the month, it sets around 9:30 p.m. and has a disc that is 10.3 seconds-of-arc across. Venus will be magnitude -3.9.
Now very poorly placed for observing, Jupiter is barely 17 degrees up in the west-northwest as it gets dark and sets by 10 p.m. It is still moving eastward in northeastern Taurus, shining at magnitude -2.0, and its disc is 32.8 seconds-of-arc across at midmonth. Mercury passes 2.4 degrees north of Jupiter on March 27 (best seen on the evening of March 26), and Venus passes 1.2 degrees north of Jupiter on March 28 (best seen that evening).
Saturn is just past opposition and is 25 degrees up in the east-southeast as it gets dark. It sets around 5:30 a.m. It moves westward from Libra into Virgo on May 13 and ends the month there. At midmonth, the Ringed Planet shines at magnitude +0.2, with a disc 18.7 seconds-of-arc across. The Rings are 42.4 seconds-of-arc across and tilted down 17.7 degrees with the northern face showing.
The Sun will be eclipsed by the Moon on May 13 for those in northern Australia and the south Pacific. This annular eclipse has a maximum duration of 6 minutes and 3.4 seconds. The penumbral eclipse on May 24 is so shallow that you will not be able to detect it even though it occurs in our morning sky. The next total lunar eclipse that we will be able to see is next year on April 15, so mark your calendar 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.