| Mar 27, 2008

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Night Skies - April 2008 Saturn, Mars, And Pleiades Occultation In Evening Sky by Leo Enright

As March ends and April begins, we are at the time of year when it is easy to notice the increasing amount of daylight from day to day. Locally on April 1, sunset is at 7:30 p.m. Eastern Daylight Time. By the end of the month it will be slightly after 8:00 p.m. – meaning a change of about one minute per day. The time of the end of evening astronomical twilight changes by even more than one minute per day, going from 9:15 p.m. EDT on April 1 to about 10:15 on April 30. The sunrise times on April mornings change even more dramatically, occurring earlier each morning by an average of almost 2 minutes per day – moving from about 6:50 a.m. EDT on April 1 to about 6:00 a.m. on the 30. With this information, you may calculate the times of local sunrise, sunset and end-of-evening-twilight for each day of the month.

On any clear evening in April, even before twilight has ended, the bright stars of the great winter constellations, that dominated the winter skies, may now be observed sinking downward toward the western horizon. I refer to Orion, the Great Hunter, with Taurus on his right, Lepus below him, Canis Major and Canis Minor on his left, and Gemini above him. At the same time, the large “spring constellations”, that is Leo the Lion, Hydra the great serpent, and Virgo the Maiden are moving up into positions of prominence in the eastern and south-eastern skies. As the night advances, the southern sky, which was occupied by Orion during winter evenings, is now filled with the stars of Leo the Lion, with its “large backwards question mark” forming the head, mane, and front paws of the huge feline, and its “right-angled triangle” of stars forming its “hind-quarters”. To the left of Leo and well up in the southeastern sky by the end of twilight is Virgo, marked by Spica, a brilliant whitish-coloured star amid two large rectangular star patterns – a formation in which it is admittedly difficult for the modern observer, lacking the very active imagination of his/her ancestors, to recognize the outline of a young lady carrying a bouquet of flowers – a starry pattern that was “easily seen” by those ancient ancestors. Though the modern observer may lack the imagination to recognize “the large pattern on the sky”, he/she can use a small telescope to find dozens, perhaps hundreds, of very distant galaxies in this part of the sky – galaxies that, like our own Milky Way Galaxy, contain many billions of stars. In fact, astronomers know the area of the constellation Virgo as “The Realm of The Galaxies”; yet, because of their incredible distances, not one of these hundreds of galaxies is bright enough to be seen with the unaided eye.

Of the five bright planets, only three of them may be seen during the entire month of April. One may possibly be glimpsed briefly in morning twilight in the first few days of the month, and one may be seen in the evening twilight in the last week of the month. The two dominant planets of the evening and night sky are Saturn and Mars. Saturn is very bright and high in the southeastern sky in evening twilight. It is just below the star Regulus, brightest star in the constellation Leo the Lion, and the planet and star cross the night sky together (as all celestial objects do because of the earth’s rotation). Careful observers will note several things: 1) that Saturn is considerably brighter than Regulus, 2) that, over the course of the month, the apparent distance between Saturn and Regulus decreases from about 3 degrees (the width of three fingers held at arm’s length to about 2 degrees (the width of two fingers), and 3) if they observe the planet with a small telescope, they can easily see the planet’s ring system and at least two of its many moons. Mars in evening twilight is high in the southwestern sky and in the constellation Gemini. The movement of Mars should be quite obvious to any observer over the course of the month, if we imagine the constellation Gemini as a set of twins standing side-by-side under the two stars that bear their names, Castor on the right and Pollux on their left. On April 1, Mars is located at the feet of Castor, and by the month’s end it will have moved up to the shoulder of Pollux. This movement which we can detect quite easily is the real orbital motion of the planet Mars around the sun. Careful observers may be able to note also that Mars fades in brightness slightly over the month, and also that, as it appears closer to the star Pollux, it becomes evident that both that star and the planet are a reddish orange colour, but one may have a slightly deeper shade of orange. Which one do the think it is, the planet or the star?

The third planet to be seen this month is Jupiter. It appears in the southeastern sky before morning twilight begins and rises higher during morning twilight. It is quite unmistakable, being much brighter than any star in the whole sky. The distant background stars to its right are those of the constellation Sagittarius with its distinctive ‘shape of a teapot’. Observers who have a small telescope may enjoy an early morning view of Jupiter an the four brightest of its moons. The planet previously mentioned as “being possible to glimpse” is Venus. Only if the weather is cooperative and the observer has an extremely good eastern horizon during the first five or six days of the month will there be a chance of seeing brilliant Venus just above that horizon for a few minutes beginning about 30 minutes before sunrise. There will be a better chance of seeing the planet Mercury in the LAST week of the month when it appears very low in the western sky for a short while beginning about 40 minutes after sunset. Be sure you have an unhindered view of the western horizon and begin looking for it every clear night beginning on April 24.

Several lunar conjunctions with bright stars and planets are well worth seeing this month. First and foremost on the list is the Moon-Pleiades conjunction in the western sky on the evening of April 8. The Pleiades (or Seven Sisters) is the most famous star cluster in the whole sky. That evening, not only does the moon appear close to it, but the moon actually occults (or moves in front of and hides) certain stars in the cluster. Use ordinary binoculars to see some of the stars disappear and others reappear as the moon moves past them. It’s an excellent way to appreciate the speed of the moon’s actual motion in its monthly orbit around the earth. A not-to-be-missed event! On the evening of April 11 the moon appears quite close to the planet Mars. On April 12 the moon appears in a line with the stars Castor and Pollux. On April 14 the moon forms a line with the star Regulus and the planet Saturn, and on the following evening the Moon appears to form a triangle with that star and planet. In the morning twilight on April 23 the moon appears to the lower right of the reddish star Antares and in the eastern morning sky on April 27 the moon appears below the very bright planet Jupiter.

A meteor shower that is usually of interest to skywatchers at this time of year is the April Lyrid Meteor Shower which reaches its peak on the night of April 21. Even though a very bright moon will prevent many of the meteors from being seen, it may still be worth observing for a while with the chance that some of the very bright ones may be seen in spite of the interference from moonlight. This shower is know, in fact, to produce some unusually bright meteors.

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.

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