SEPTEMBER STAR MAP |
- 210 SEPTEMBER 3, 2000:
When the Moon Does Not Rise or Set
- A curious circumstance occurs when our Moon is near its first quarter or last quarter phases. This is when the Moon appears to be half illuminated by the sun. Near first quarter, there will be a night during which the Moon will not rise. Last quarter moons will bring an evening when the Moon does not set. This week the Moon will not set on Wednesday. On Tuesday the Moon is at first quarter. Likewise, on September 21st the Moon will not rise. The Moon is at last quarter on September 20th. This phenomenon has to occur around the quarter moons, because it is at this time that the Moon either rises or sets near midnight. The reason why there is a night when the Moon does not rise or set stems from the fact that two successive moonrises or moonsets transpire at intervals which are on average 24 hours 53 minutes in length. This a result of the daily eastward motion of the Moon against the starry background as our nearest neighbor in space orbits Earth. As an example, the first quarter moon sets at midnight on Tuesday. The Moon will next set again in 24 hours 42 minutes. This will not allow enough time for the Moon to set on Wednesday. So moondown will occur early on Thursday at 12:42 a.m. Likewise at last quarter, on September 23rd, the Moon does not rise until 11:41 p.m. It cannot rise the following day because the cycle between rises at this time is 24 hours 57 minutes. So there is no moonrise on September 21st, but the Moon does rise early on the 22nd at 12:38 a.m. In October the Moon will not set on the 5th and will not rise on the 20th. These correspond to the dates of the first and third quarter moons.
- 211 SEPTEMBER 10, 2000:
Where Have All The Planets Gone?
- Where have all the planets gone? Last spring, they abounded in the evening sky, but with the warmer months, they seemed to disappear amid the glitter of brighter summer stars. Now that fall is nearly upon us, they will be ever so gradually reappearing. Patience, persistence, and a little insomnia will help in your quest if you want to see them now. The bright Harvest Moon isnít helping matters either, although tonight you may want to note the Moonís position in the sky around 10 p.m. Uranus will be only three lunar diameters above the location of the Moon in the constellation of Capricornus, the Sea Goat. Neptune is to the west. Youíll find a map of this zodiacal constellation, which is located just east of Sagittarius, in the StarWatch section at the web address below. The Goat was also featured two weeks ago in StarWatch. Venus is visible in the west. It began appearing after sundown in early July, but it is still a challenge to find because Venusí orbital path lies at a shallow angle to the horizon. It sets about one hour after sunset. Go out at 7:30 p.m. about 30 minutes after sunset. Use binoculars. Venus will be about one binocular field above the WSW horizon. Youíll need an exceptionally clear evening and unobscured horizon. For the easy, but late night planets, look east between midnight and 2 a.m. Youíll see three bright "stars," which form an isosceles triangle. Two of three luminaries will be planets. Aldebaran is at the lower right. Slightly above and to the left will be Jupiter, and above them all will be the ringed planet Saturn. Higher still will be the Seven Sisters or Pleiades, a sure harbinger of fall.
- 212 SEPTEMBER 17, 2000:
Where is My Sunshine Going?
- At 1:27 p.m. Friday the sun reaches a crossroads in its path among the stars. At that moment the very center of the solar disk intersects the Celestial Equator and fall begins. Also called the autumnal equinox, it is one of four milestones in the sunís apparent motion in the sky due to the orbital cycle of the Earth. The entire world except for the extreme polar regions is treated to a 12 hour period in which the sun is visible and a 12 hour period in which it is not. At the North Pole the sun is setting heralding a 6 month period of absence. At the South Pole the sun is rising to be visible for a six month period. The equinoxes also represent the period of greatest changes in the sunís altitude and in its length of visibility. We, in the Lehigh Valley, are losing nearly 3 minutes of sunlight each day. In one week we will have lost 19 minutes, in two weeks, 37 minutes, and three weeks, 56 minutes. Keep in mind that we lose an equal amount of time in the three weeks prior to the autumnal equinox. Thatís nearly 2 hours in the past six weeks. No wonder these differences are so apparent. However, it gets worse as you travel farther north. I spent several days in Fairbanks, Alaska this past August and can attest to the fact that it never got dark. However on September 22nd, Fairbanks will have 12 hours of day and 12 hours of night. On the 23rd the sun will shine for 7 minutes less in Fairbanks. One week later it will shine for 46 minutes less, two weeks later, 93 minutes less, and three weeks later, 139 minutes less. During the six weeks surrounding the equinox, Fairbanks loses 4 hours 38 minutes of sunlight. With changes like that, Hawaii and southern California get to look very inviting by mid-January.
- 213 SEPTEMBER 24, 2000:
Milky Way Number One
- It all started with a galaxy not so far away. The galaxyís name is Andromeda, a beautiful spiral that can been seen with the unaided eye from our rural Valley skies in the fall. There was also a very short magazine article in the October 2000 issue of "Sky and Telescope" that would forever change my thoughts about our local area of space. For decades the Andromeda galaxy had been pegged at 2.1 million light years from our Milky Way. A light year is the distance that light travels in one year, about 5.8 trillion miles. The Hubble Space Telescope changed that number to about 2.6 million light years. I just recently began using that figure in my astronomy presentations. Our Milky Way galaxy and Andromeda are the two key players in what astronomers call the Local Group. It is an assemblage of about 35 galaxies held together as a unit by gravity. Up until this article, the standard number of galaxies in our Local Group was 21. Ever since the movie "Contact," the Milky Way was listed as having about 400 billion stars. Andromeda was bigger with perhaps 600 billion luminaries. This meant that our Milky Way had something in common with Avis car rentals. We were both number two. Not so according to this aforementioned article. Although the Milky Way has fewer stars, it has about two trillion sunís worth of dark matter in addition to those stars. The Andromeda galaxy has only 1.2 trillion sunís worth of this unseen stuff. You do the math. Milky Way is now number one and by a rather impressive amount. Our galaxy has become the biggest fish in an even larger fish bowl. Finally, what I knew all along had become the truth. Our passion with size and being first had extended itself clear into the universe.