August 6, 2023
Saturday, August 6, 2023
Tonight, the moon is in a waning gibbous phase, two days before third (or last) quarter. That means it will be just a waning crescent phase this coming Saturday night (the night of August 12/13, rising at about 3:30 a.m. among the stars of Gemini. What’s special about this coming Saturday is that it is the peak of the Perseid meteor shower. Having the moon at just 8% illumination and rising so late means that the moon is not going to wash out a good showing from the most popular of meteor showers.
In my previous column I talked about the connection between comets and meteor showers but here’s a refresher. Comets spend most of their time far from the sun as frozen dirty icebergs about the size of a small town or mountain. This dirty iceberg, called a comet nucleus, is made of various types of ices such as water, carbon dioxide, carbon monoxide, etc. and dust and rock bits are embedded in those ices. When the comet nears the sun, the nucleus warms up and the dust and rock bits are released, often explosively in jets. The comet nucleus’ gravity is much too weak to hold onto the released dust and rocks, so they are left along the comet’s orbit.
The Perseids result from Earth plowing through the dust trail left behind by Comet Swift-Tuttle. The nucleus of Swift-Tuttle is on the larger side as far as comet nuclei go, about 26 kilometers across and the dust trail is quite broad. Earth has been going through the dust trail since mid-July and we won’t get out of the dust trail until the end of the month. That means, of course, that you’ll be able to see Perseid meteors tonight, a week before the peak and you’ll have a chance to see them well after the peak, though the moon is going to be a problem in the latter part of the month.
The combination of Earth’s motion around the sun and the dust/rock speeds in their orbit around the sun mean that the Swift-Tuttle bits hit our atmosphere at about 37 miles per second (or about 133,000 mph). At such a speed, the comet bits (most about the size of a grain of sand) will burn up several tens of miles above the surface and also cause the air immediately around the comet bit to ionize and glow briefly. We see that as a streak of light and the larger rocks can make fireballs (long-lasting meteors that are brighter than all of the planets, except Venus, though some can get much brighter than Venus). The Perseids tend to make more fireballs than any other meteor shower.
If you have a nice dark sky far from the city, you might see up to 70 to 90 meteors per hour on the night of the peak. On the edge of Bakersfield, you’ll see at most just half that amount but that’s still a good show. Some nearby good observing spots include Wind Wolves Preserve, the Frazier Park area, and the high desert area east of Tehachapi — get up out of the valley!
Venus is now out of the evening sky, leaving Mercury to be the evening star, shining low in the west for the next couple of weeks. Venus will be at “inferior conjunction” between us and the sun on August 13. This line-up happens every 584 days but because Venus’ orbit is tipped or inclined with respect to our orbit, Venus passes either above or below the sun as seen from our vantage point. In fact, the chance that Venus will be lined right up with the Earth and sun at the inferior conjunction is extremely small. Transits of the sun by Venus happen less than once a century. The last Venus transit was in June 2012 and the next one will be December 2117. I’m glad that my wife and I got to see the transit from the top of Mauna Kea because we knew that we were not going to see that again in our lifetime!
Mercury is getting closer to Mars and between August 10 and 16, they’ll be close enough to fit within the same field of view of your binoculars. The next pairing will be August 18 when a thin waxing crescent moon visits Mars. They’ll be just a thumb width at arm’s length apart, so they’ll be a beautiful sight in your binoculars as well as just your eyes.
If you’re out after midnight observing meteors, you’ll be able to see the two giants of the solar system, Jupiter and Saturn. Saturn first becomes visible at about 9 p.m. and is much brighter than the stars in Aquarius in which it resides for the next year or so. Jupiter becomes visible just after midnight next to the head of Cetus. Jupiter will be brighter than any star in the sky and Saturn will be brighter than all of the stars except Capella and Vega.
Enjoy the meteors!
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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
