Bakersfield Night Sky — July 21, 2024
Mercury is still visible low in the west in the evening, a fist width at arm’s length above the horizon at 8:30 p.m. but it is quickly losing altitude over the next few days. It will be gone from the evening sky, lost in the evening twilight glow before the end of the month. The moon was at full phase this morning, so it will spend the rest of July shrinking in the amount illuminated.
That amount illuminated is as seen from Earth, of course. The moon is always half illuminated by the sun but the amount of that illuminated part that faces Earth changes as the moon orbits Earth. We see the terminator boundary between the day and night sides of the moon slide across the moon (from right to left when viewed in the northern hemisphere) as the days (nights) go by. The fact that the moon goes through phases showed the ancient Greeks (and probably others as well) that the moon does not produce its own light but must be reflecting sunlight. The moon also keeps one side facing us because tidal gravitational forces of Earth have slowed the moon’s rotation period to be the same as its orbital period. Although there is no perpetual dark side to the moon, there is certainly a perpetual far side to the moon!
On the nights of July 23 and 24, we’ll see the moon and Saturn rising together around midnight. The moon will rise before Saturn on July 23 and after Saturn on July 24 with about half a fist width separating them on each night. On the night of July 27, the moon will be at third (last) quarter, the left half side appearing lit from our vantage point. At the end of the month, on July 30, the waning crescent moon will be near Mars and Jupiter, creating a squarish pattern with them and the bright star of Taurus, Aldebaran. They will all be visible after about 2:45 a.m. On the last morning of July, the moon, Jupiter, and Aldebaran will make a line on the sky with all rising at the same time.
As I was picking up trash during a walk around my neighborhood, I thought about how I distinguished human-made trash from the natural debris of leaves, sticks, rocks, etc. and how my search was similar to the search for signals from extra-terrestrial intelligent civilizations that want to make themselves known. Human trash is monochromatic with straight and sharp well-defined edges while natural debris is not. For example, a lollipop stick is solid white, straight, and smooth while a tree twig is going to have multiple shades of brown and gray with all sorts of kinks in it. A candy wrapper is shiny with a few bright colors while a leaf or flower petal is softer with a spectrum of colors.
Celestial objects emit radiation over a wide range of wavelengths. Even the emission lines from rarefied gas clouds have a rounded though narrow spectrum. Celestial objects will not be perfectly reflective, at best behaving like a very dirty mirror, and the edges will be a bit fuzzy. If they change their brightness, the brightness will ramp up and then ramp down over time and each successive peak won’t be exactly the same as the one before.
If ETs want to make themselves known, they will produce signals that are monochromatic, exactly repeatable, and will suddenly turn on and off. They will create objects with surfaces that are glassy smooth surfaces over a large scale or large-scale structure with straight, well-defined edges. There are exceptions, of course, to what natural debris is like. For example, obsidian is glassy smooth and can have well-defined edges. When pulsars were first discovered, they sure looked artificial because of their precise repeatability. Now we know that pulsars are the collapsed, super-compressed cores of formerly massive stars that are spinning very rapidly with super-powerful magnetic fields that can produce beams of light that might sweep over Earth as the pulsar spins if they’re lined up right. However, the exceptions will be rare by definition and if we can narrow the search down from billions to trillions of objects to several thousand, that will make the search manageable and practical for follow-up observations to look for other artificial vs. natural characteristics.
I hope that you will be able to find a place this summer that has a nice dark sky and enjoy a night sky filled with stars!
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Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com
