Night Skies June 30/04
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A Brilliant Planet in Evening and Morning Sky
The mild nights of July are often occasions for many people to study the starry sky and to become better acquainted with the myriad of stars and the many constellations that are so easily seen at this time of year. Letting your gaze drift along the Milky Way in a great venture of discovery can happen on any clear July night when moonlight does not interfere and when interested people can get away from the unwanted glare of streetlights and other forms of manmade light pollution. With the very long twilights at this time of year, many serious skywatchers begin their observing before twilight ends in order to watch as thousands of stars pop into view, one by one, in order of their brightness. Locally, in July sunsets are between 8:45 and 8:25 p.m. EDT, and the end of astronomical twilight ranges from 11:15 to 10:50 p.m. over the course of the month. (No, those times are not written backwards. Sunset and end of twilight times are occurring earlier from day to day as the month passes, since we are now past the day of the latest sunset of the year.)
Late in the evening of any clear July night is the ideal time to study the constellation Sagittarius low in the southern sky. Its distinctive star pattern reminds the modern observer, not of a warrior carrying a sheaf of arrows, but of simple teapot. With its distinct handle to the left and its spout pointing westward, that is, toward the right, this large Teapot of Sagittarius seems to pour out its nourishing contents, as it tips and moves forward over the course of a night. The glowing band of the Milky Way is thickest in the area of this southern constellation, and at the place where the Milky Way is most concentrated, that is, to the right of the teapots spout, the centre of our Milky Way Galaxy is located, about 40,000 light-years away. As your eyes follow the sweep of the Milky Way upwards from Sagittarius and toward Cassiopeia, the large W-shaped constellation in the north-eastern sky, try to remember that that great river of starlight is actually coming from many millions of the 200,000,000,000 stars that make up our galaxy, and that our own Sun is just one of those countless millions of stars.
This month all five of the bright naked-eye planets can be seen at various times, but one of them dominates the evening sky and one rules the morning sky. The brightest planet of all, Venus, totally dominates the morning sky, as it will for the next eight months and as it did in the evening sky for the nine months before its transit across the sun on June 8. I was extremely fortunate to enjoy good weather conditions on that morning, and from my home, was able to observe the event from 5:31 a.m., 10 minutes after sunrise, until about the time of the Venus egress from the sun at 7:26 a.m. Using the telescope and solar filter with which I regularly observe sunspots, I was able to observe the movement of Venus on the solar disk for almost two hours and to photograph this event, which had not been witnessed since December 1882. The next two occurrences of a Venus Transit are in June 2012, and in December 2117. Since then, my first sighting of Venus in the morning sky was at 4:45 a.m. on June 20 when it could be seen for only about a half-hour between its rising and sunrise. Throughout July, the time for seeing Venus increases from about 90 minutes to over 3 hours between its rising just north of the east point on the horizon and its being lost, or almost lost, in the glare of the rising sun. However, it is so amazingly bright this month that observers will be able to follow it throughout the day! It reaches its maximum brightness at mid-month, July 15.
Just as Venus dominates the eastern morning sky, Jupiter, the largest and the second brightest of the planets, dominates the western evening sky. In early July it can be seen well up in the west in early twilight, and setting over the next three hours. Later in the month, it can be seen for less than two hours after sunset. For those with a good view of the western horizon, Mercury is easy to see beginning about 40 minutes after sunset. Look very low in the sky and a bit north of the west point on the horizon to follow this planet for more than a half-hour before it sets. Mars, which is reddish in colour but fainter than Jupiter and Mercury, is also low in the west and best seen starting 40 minutes after sunset, though binoculars will help because it is in bright twilight and much fainter than it was last summer. Before July 10, Mars will be slightly up and to the left from Mercury; after that date it will be slightly down and to the right from Mercury. On the night of July 10, these two planets will appear extremely close to each other in an event called a planetary conjunction. Try not to miss it in your binoculars, if possible, that evening beginning 40 minutes after sunset. (Of course, Mars and Mercury are not physically close to each other; it is just a matter of alignment. Mars is actually over twice as distant as Mercury.) The planet Saturn, whose orbit this month has taken it around to the far side of the sun, is not easily seen until about July 23 when it may be seen rising above the northeastern horizon about an hour before the sun rises.
On several days this month the moon presents interesting sights in conjunction with various planets. In the eastern sky an hour before sunrise, try to notice the waning crescent moon glide past Venus on the mornings of July 13,14 and 15. Dont miss trying to see the very slim waxing crescent moon low in the west near Mars and then Mercury about 40 to 60 minutes after sunset on July 18 and 19. Much less challenging will be seeing the larger crescent glide past Jupiter, also in the western sky, at the same time on July 20 and 21.
Another solar system fact for this month, one that is little appreciated by most people, is that on July 5 the earth will be at aphelion, the point in its orbit at which it is most distant from the sun. The earth-to-sun distance will be about 5 million kilometres greater than at the perihelion (closest earth-sun distance) which occurred on January 4. Of course, very obviously (!), this change, which amounts to only a little over 3% of the total distance, is not one to affect climate or weather, since there are more immediate factors that affect those conditions on our planet. Also, as a result of this, although the sun appears smaller at this time of year than in January, that change in its apparent size is so very minimal that almost no one would ever notice it.
Doubtless this month, there will be some people mentioning the fact that a second Full Moon of the month occurs on July 31, and they will repeat some lingo about it being called a blue moon, as if it had some astronomical importance. This phrase is a fairly recent creation, and of no astronomical relevance, other than recognizing a fact of counting days. Since the length of the synodic month (the time between similar moon phases) is 29.53 days and since there are many months of 31 days, then there are frequently months in which one of the lunar phases is repeated.
A newly revised book called The Beginners Observing Guide is now available at the Valley Book Shop in Perth and at Sharbot Lake Pharmacy.
