Starry Skies Over Santa Monica

July 21-28

Mirek Plavec, Emeritus Professor of Astronomy, UCLA

Planets
   Two bright planets remain easily visible in the evening sky, but their nights of glory are slowly coming to an end. Venus is still brilliant and easy to see shortly after sunset above the western horizon, and is still fairly close to the first-magnitude star Regulus in Leo. However, as I attempted to explain last week, Regulus sets 4 minutes earlier every evening, and Venus is slowly sinking with it. Be sure to look for Venus before 9 p.m.
   Mars is fairly high in the south when the sky gets darker. Although fading, it is still brighter than the first-magnitude star Spica, from which it slowly recedes eastward. Mars is now in the inconspicuous constellation Libra.
   The Moon is the real attraction this coming week. It will be Full on the night of July 27/28. It is quite exciting to watch the full moon rise, if you can see it rise above the distant horizon, not above your neighbors chimney! If seen rising from behind some distant mountains, the Moon appears unbelievably big! Later on, when it is much higher in the sky, it looks very much smaller—yet, at this later time the Moon is a little bit closer to you than it was at the rising time.
   How come? The explanation is that your eyes conspire with your brain and are cheating on you! When you look at a distant mountain or tower, the brain tells you: that object must be real big when you can see it at such a great distance—and then you begin to see it as big! And when the Moon joins the show, your brain applies the same reasoning and you see an unusually large disk of the Moon. It will shrink perceptibly if you prevent this conspiracy between your eye and your brain. It suffices to look at the Moon through a very narrow opening in your nearly-closed fist, or through a long narrow tube. You will be surprised how small the Moon is! Another action that might work in the same way is to turn your back to the Moon, bend deep forward, and try to watch the Moon in between your legs. I say that it might work because I am unable to perform one of the steps described above. However, I think that even if you succeed and may not be sufficiently satisfied by your observation, people watching your gymnastics may be quite impressed.
   A partial eclipse of the Moon will occur on that Full Moon night, from Tuesday July 27 to Wednesday, July 28. Actually, the eclipse comes in the wee hours of Wednesday. The shadow of the Earth will encroach on the disk of the Moon at 3:22 a.m., the eclipse will be largest at 4:34 a.m., and the shadow will leave the disk of the Moon by 5:46 a.m.
   By that time, the Moon will already be very low in the south-west, and the Sun will rise some 15 minutes later. No more than about 40% of the disk of the Moon will be eclipsed at the peak of the eclipse, so I am not sure if I should recommend to you to observe the eclipse. However, if you happen to wake up during the eclipse, by all means, go and find the Moon low in the south-west!
   The eclipsed part of the Moon's disk will be much fainter than the rest of its disk, but most likely, you will have no problem in seeing it.    Even if the Moon is totally eclipsed, it usually remains quite easily visible, although it acquires a rather unusual brownish or ruddy hue.    This should actually be surprising, since the Moon has no light of its own, and, during totality, it is fully immersed in the shadow of the solid Earth. The culprit is our own atmosphere. The rays from the Sun are somewhat curved when passing through the air, and fall on the Moon; so the Moon does get some fraction of sunlight. OK, what about the color? Actually, only red and orange rays make it through our atmosphere. The photons of shorter wavelength, especially those violet, blue, and green ones, are filtered away during the long path through our atmosphere. At the time when we have a total eclipse of the Moon, an astronaut standing on the Moon would see a total eclipse of the Sun by the Earth. The Sun would disappear completely (except perhaps for the edges of the solar corona) behind the much larger, dark Earth, but the dark disk of the Earth would be surrounded by a beautiful reddish aureole —this is the sunlight filtering through our atmosphere and falling on the Moon!
   Our atmosphere is not always equally transparent. At times, there could be so many clouds at the critical levels that the Moon gets very little sunlight, even of those red and orange colors. Among some 12 total eclipses of the Moon I have observed , one was quite exceptionally dark. I was standing above the cliffs in Pacific Palisades, I knew exactly where the eclipsed Moon should be among the stars—and I had a very hard time to find it! There were eclipses in the past when the Moon disappeared completely, but this happens very rarely. More often, I was a bit disappointed that the Moon was too bright! We will see what happens next year—we will have two total eclipses.
   Actually, why are the eclipses of the Moon so rare? After all, it stands opposite to the Sun every month, at the time of full moon! The reason is that the Moon does not revolve about the Earth exactly in the same plane in which the Earth orbits the Sun. The plane of the Moon's orbit is tilted by about 5 degrees, which means that, more often, the Full Moon passes either above or below the shadow of the Earth, and no eclipse occurs. But the orbit of the Moon crosses the plane of the Earth's orbit twice, at so-called nodes. The Moon passes through both nodes once in each revolution about the Earth, but for an eclipse to occur, the Moon must also be full (then we have a lunar eclipse) or new (then it will eclipse the Sun). We are having one eclipse season now, with the partial lunar eclipse in July, and a total solar eclipse in August. The next such season comes in January, with a total eclipse of the Moon visible from California on January 21, 2000.
   Would you like to see the shadow of the Earth anyway, and not wait for an eclipse? Easy! Wait for a clear, cloudless evening, and look towards the eastern horizon soon after sunset! You will see a huge, dark blue arc slowly rising above the horizon and this is the shadow of the Earth upon our own atmosphere!