Jeff Green | Apr 02, 2009
Back to HomeNight Skies - April Several great lunar-planetary conjunctions
by Leo Enright
On April 1 sunset locally is at about 7:30PM EDT, and on the last day of April sunset is slightly after 8PM – an average change of about one minute per day. For those skywatchers who wait in the evenings for the end of astronomical twilight, its changing times for the month are even more noticeable, moving from 9:15PM on April 1 to 10:15PM on April 30; that is, a difference of 60 minutes in the month, or an average change of two minutes per day. The sunrise time changes in the morning are even more noticeable than the sunset times. Sunrise at the beginning of April is at about 6:50AM, and at the end of the month at 6AM, a change of 50 minutes, or a bit less than two minutes per day.
As the sky darkens each night in April, it is easy to observe that the constellations with which we became familiar over the winter months – with their many brilliant and gleaming stars – are now sinking into the southwestern sky and setting as twilight ends, and are being replaced in the southern and eastern sky by what astronomers call “the stars of spring” – mainly found in three very large constellations that will dominate the early night sky for the next three months -- Leo the Lion, Hydra the Water Snake, and Virgo the Maiden. These spring constellations do not have as many brilliant individual stars as the winter constellations, but they do claim their distinction because of their great size. Of all the 88 constellations in the whole sky, two of these three are the very largest. Hydra is the very largest constellation, covering 1303 square degrees of sky, and Virgo is second covering 1294 square degrees. Leo the Lion is also very large, but it is beaten out for third place by the northern sky’s great bear, Ursa Major, which contains the Big Dipper.
Leading the spring-time parade of stars across the sky during the nights of April is Leo the Lion whose arrangement of stars looks like a huge backwards question mark – showing the head, the mane, and the forepaws of the lion. A right-angled triangle of stars marks the beast’s hindquarters. Its brightest star located at the bottom of the backwards question mark, is what was called in ancient times “the King Star” or “Regulus”, which marks the front paw of the feline. Following Leo is Virgo, the Maiden, though the modern observer, with a less active imagination than their ancient counterpart, usually finds it difficult to distinguish the shape of a young lady carrying a bouquet of flowers. He or she usually sees only two large rectangles with the brilliant white star called Spica almost in the middle. Hydra, the huge Water Snake, has its serpentine head below and to the right from Regulus in a cluster of stars that can be seen with the unaided eye for a rural site, and its long string of stars includes the bright reddish one called Alphard located well below Regulus. The creature’s very long slithering body stretches far to the left and down to the horizon far below the star at the end of the triangle that marks the tail of Leo. In fact, as seen at the latitude of southern Ontario, the tail of Hydra extends right down to the southeastern horizon, and then curls back up and wanders further to the left below the entire width of the constellation Virgo. Our discussions here have mentioned only three constellations, but they have included an enormous portion of the sky as seen at this time of the year. Readers are urged to become familiar with them and with the brightest star within each one of them, as named above, because these constellations may be mentioned again over the next two months.
Of the five bright planets that may be seen with the unaided eye, three of them may be seen in early April, but for the first time in many months, we can say that ALL FIVE planets may be seen in late April. We should try to take advantage of this opportunity that has not occurred in a very long while. Bright Saturn comes into view early in evening twilight and remains visible all night. It is seen just below the triangle of stars that form the “hindquarters of the Leo the Lion” (See above), and it will remain among the stars of this constellation for several months, but it will be easily distinguishable from them because it is brighter than any of the stars of Leo, including Regulus. Very careful observers will see that Saturn is in the retrograde portion of its orbit and is moving slightly westward toward the star Regulus. A small telescope will reveal one or more of its many moons and its system of rings. From April 16 to the end of the month, the planet Mercury may be easily seen by those who have a good view of the western horizon and look low to the west 30 to 60 minutes after sunset. Jupiter is easily seen in the eastern sky, rising in the morning about one hour before sunrise in early April, and earlier each morning until by the end of April it is rising over three hours before sunrise. Except for Venus and the moon when present, Jupiter is the brightest object in the eastern morning sky. Venus is amazingly brilliant throughout April, but may require some effort for those who have seen it so effortlessly in the western evening sky for the past eight months. It is now low in the morning sky and will require a horizon unobstructed by buildings or trees. It rises about 45 minutes before sunrise at the beginning of the month, then a slightly later rising each morning, and one hour and 45 minutes before sunrise at the end of the month. The view of Venus in a small telescope, over the course of the month, will reveal a change in phase from a slim crescent to what resembles the “Last Quarter Moon”. The red planet, Mars, is also seen low in the eastern morning sky during April, but it is not nearly as bright as Jupiter or Venus. In the latter half of April, Mars may be seen below Venus by those who look carefully between 60 and 30 minutes before sunrise. Binoculars may be helpful in spotting Mars.
Several lunar-planetary conjunctions are worthwhile celestial attractions this month. Watch the waxing gibbous moon on the evenings of April 4, 5, and 6 as it moves through the constellation Leo the Lion. On the 4th it is to the right of Regulus. On the 5th it is just below Regulus, and on the 6th it is just below Saturn. Low in the southeast on the morning of April 19, the waning crescent moon is beside Jupiter. In the same part of the sky on the 20th and 21st it is between Venus and Jupiter. Then on the morning of the 22nd, don’t miss the view in the same part of the sky as the slim Crescent Moon is right beside Venus. Set the alarm for 50 minutes before sunrise so as not to miss that view of the night sky’s two brightest objects so close together in the morning twilight. In the evening twilight on the 26th also try to catch the view in the west of the slim waxing crescent moon right beside the famous Pleiades Star Cluster and just above the planet Mercury – best seen about 40 to 60 minutes after sunset. Again use binoculars if the Pleiades are not seen with the naked eye.
More information about observing the spring and summer sky is available in the book entitled “The Beginner’s Observing Guide” which is now available at Sharbot Lake Pharmacy and on the internet at www.rasc.ca/publications.