Bakersfield Night Sky - September 16, 2018

Bakersfield Night Sky - September 16, 2018
by Nick Strobel

The first show for the fall schedule of evening shows at the William M Thomas Planetarium at Bakersfield College will be this Thursday, September 20. “From Dream to Discovery” will give an inside look at the years of engineering design, building, and testing that it takes to create the spacecraft that travel to the planets or peer out to the farthest reaches of space. Two weeks later, on October 4, will be “Supervolcanoes” which is about the largest possible volcanic eruptions with energies and amounts of volcanic ash many thousands of times greater than the Mount St Helens eruption of 1980 or Pinatubo in 1991. As I write this, tickets are still available for both shows. See the Planetarium's website for more details about the shows.

Two days after the “From Dream to Discovery” show, or September 22 is the autumnal equinox when our season of autumn or fall officially begins. This happens when the geometric center of the sun's disk crosses the celestial equator heading southward. Although this is supposed to be when the day is equal to the night, the time of daylight does not use the geometric center of the sun's disk for its measurement. Instead, daylight begins when the topmost edge of the sun becomes visible above the horizon and ends when the topmost edge of the sun disappears below the horizon.

Since we (thankfully) live on a planet with an atmosphere, atmospheric refraction (bending of light as it passes through our air) causes the sun's disk to appear higher in the sky than it would if Earth had no atmosphere. You can notice the effect of atmospheric refraction most easily if you observe the sunrise or sunset at a place with a flat horizon such as at the coast. The sun appears like a squashed ellipse instead of being a nice round circle. Putting both effects of measuring daylight from the top edge of the sun and atmospheric refraction together means that the twelve hours of daylight is actually going to be several days after the autumnal equinox, on September 26.

The day I wrote this column was a week ago, September 9, when the moon was at “New Moon” phase. New Moon marks the start of the lunar phase cycle and it is when the moon is lined up with sun. All we see is the night side facing us. As the moon moves around Earth, the moon appears at a larger and larger angle from the sun on our sky and more and more of the day lit side of the moon faces Earth up through full phase, when the moon and sun are opposite each other in the sky. After the “Full Moon” phase, the angle between the moon and sun shrinks and we see a decreasing amount of the day lit side of the moon facing us.

The Mutts comic strip for September 9 showed the dog and cat main characters looking at the New Moon. One was pointing out the “New Moon” and the other wondered “what was wrong with the old one?” Now, I do understand the humor in that line but the astronomer in me noticed that there was a white circle around the black moon and the moon was in a night sky with lots of stars. Since the New Moon is lined up with the sun, the moon would be visible above the horizon only during broad daylight and you would have to look right next to the sun—a difficult thing to do (and also a difficult thing to draw for a Sunday comic strip).

Tonight the moon is at first quarter phase, when the moon is one-quarter of the way around in its orbit and the moon makes a 90-degree angle with respect to the sun on our sky. We see one half of the moon's day lit side and one half its night side. On September 24, the moon will be at full phase. Each of the major phases (new, first quarter, full, and third quarter) are about seven days apart from each other which is probably why our week is seven days long (and not six, eight, or ten days). The whole lunar cycle is 29.5 days long which explains the length of the time period we call a “month”.

Venus has been getting harder to see as its altitude after sunset decreases over this month. Venus has “rounded the corner” in its orbit and is now drawing nearer to passing between the sun and us. As it does so, we see less of its day lit side, so it looks like a thinner and thinner crescent. Although, the fraction of the day lit side facing us gets smaller, it is also getting closer to us and therefore, getting brighter as a result. Mars continues to be very bright for the rest of the month but it is dimming as we pull away from it in our faster orbit around the sun.

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Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com