Nightskies_09-17

Jeff Green | Apr 30, 2009

by Leo Enright

In the month of May, the daylight hours continue to increase and hours of darkness continue to decrease, but not by as much, on average, as they did from day to day during the month of April. Locally sunset on May 1 is just after 8PM EDT, and by the end of the month it is at 8:40PM, an average difference of over one minute per day. Over the month, the end of evening astronomical twilight moves from 10PM to about 11PM, an average difference of about two minutes per evening. This means, as many skywatchers have learned during May in years past, there is a very noticeably longer wait for the sky to become completely dark at this time of the year. In the morning, on May 1, astronomical twilight begins locally at 4AM, and sunrise is at 6AM, but by the end of May, astronomical twilight begins just after 3AM, and sunrise is at 5:30AM. The hours of complete darkness are indeed getting to be very few. As can be seen from these numbers, complete darkness in this area on the last day of the month lasts only from 11PM to 3AM, that is, for only four hours out of the 24 hours in the day. Use the above information to calculate the times of local sunset, end of evening twilight, beginning of morning twilight, and sunrise for each night of the month.

May’s long evening twilights give is a wonderful opportunity to observe the stars, bright and faint, within the spring constellations, to which we were introduced last month, as they gradually pop into view in the southern sky. As twilight deepens, beginning about a half-hour after sunset, take note of the emerging pattern of the Leo, the Great Lion of ancient mythology, high in the southern sky and of Virgo, the young lady of the ancient stories, as she becomes evident at mid-height in the southeastern sky. As for Leo, the distinctive pattern is twofold: a ‘backwards question mark’ above the very bright star Regulus, and a right-angled triangle of star to its left, with Denebola as the brightest star in the triangle. Within this pattern our distant ancestors saw the shape of a crouching lion facing westward in the sky. Well to its left, in the middle of the reclining outline of Virgo, is the brilliant white star Spica, seen by the ancient Romans as an ear of corn or wheat held by Astraea, the goddess of justice, though people of many ancient cultures saw it as the central star in another one of their important female deities. Just as we were able to do when observing some of the brightest of the ‘winter stars’, when looking at these three bright beacons of light, try to remember that they are at vastly different distances from our Earth. Regulus is 69 light-years away. Denebola is 40 light-years from us. Spica, though it is easily seen to be much brighter than Denebola, is actually six times farther away, at a distance of 275 light-years. The next time you see Spica gleaming in the southeastern evening sky, just remember that astronomers have calculated that its INTRINSIC brightness is incredibly great – actually 2300 times as great as that of our sun. Imagine how bright it might appear if it were at the same distance as either Regulus or Denebola! Of the five bright planets that may normally be seen with the naked eye, four can be seen during the entire month and one of them can be seen during the first five days of May. For anyone who has never seen our solar system’s innermost planet, Mercury, there is an excellent chance to locate it in the first five days of May, if the observer has binoculars and a good view of the western horizon in the first hour after sunset. At that time, low in the western sky, you should easily be able to locate the star cluster called ‘The Pleiades’ or ‘Seven Sisters’ in your binoculars and to see Mercury just two degrees to the left – that is, twice the width of a fingernail held at arm’s length. Unfortunately, Mercury moves quickly and by May 3, it will move away from the Pleiades and within a couple of days disappear from the evening sky. Saturn dominates the western evening sky for the entire month. It is easily found below the bright stars of Leo the Lion (see above), the constellation within which it is found for most of this year. Those who observe it with a small telescope will see its rings slightly more open than in previous months this year. Bright Jupiter, our solar system’s largest planet, rises in the east at about 3AM on May 1, and a bit earlier each night until it rises at about 1AM on May 31. It is seen in the eastern sky for the rest of the night. Those who observe it with a small telescope can enjoy a very special treat on the morning of May 17 between 4 and 5AM when the shadows of two of the moons of Jupiter may be seen crossing the planet at the same time. (Seeing shadows of Jupiter’s moons crossing the planet is very common, but seeing simultaneous crossings is NOT common.) On the morning of May 20, Jupiter, from our perspective, will appear very close to an extremely distant background star, one that may make some observers think they are seeing a fifth moon of Jupiter, but remember that it is a distant star, and only four of Jupiter’s moons are bright enough to be seen in amateur telescopes. The planets Venus and Mars are seen this month very low in the eastern sky beginning about an hour before sunrise. There will be absolutely no question about which is which. Venus is brilliantly white and 160 times as bright as Mars. Mars is slightly reddish and to the left of Venus. The view of these two planets will be especially interesting on May 20, 21, and 22 when the crescent moon will join them. On the 20th, the moon will be above Venus; on the 21st it will be above Mars; and on the 22nd it will be to the left of both of them. For these three mornings, be sure your view of the eastern horizon is unobstructed by trees or buildings, and remember to start observing about 90 minutes before sunrise. A few days earlier, during the night of May 17, the Last Quarter Moon rises very close to the planet Jupiter at about 2AM and its apparent position remains close to that planet during the remainder of the night. Regarding the length of what is called “the sidereal month” we have a good chance to learn it this month when the moon passes two objects TWICE. Try to observe BOTH events. On the night of May 2, the moon can be seen passing near the star Regulus, and doing the same thing again on May 29. On the night of May 3, the moon may be seen passing near the planet Saturn, and doing the same thing again on May 30. (From observing these four events, can you determine, with a fair amount of accuracy, the length of a ‘sidereal month’?)

The Eta Aquarid Meteor Shower reaches its peak in early May each year, with the absolute peak occurring this year before dawn on May 6. The best chance of seeing bright ‘shooting stars’ will be had by those who look toward the southeast or east before and after the beginning of morning twilight.