Jared May: What's Up, October 22 - 29, 2021
What’s Up This Fourth Week of October
The week ahead is, unfortunately, looking pretty gloomy and cloudy. But of course the forecast is always uncertain when predicting the weather a week in advance – so maybe we will get lucky with some clear nights towards the end of the upcoming week. The average temperature of the Ohio fall nights is slowly dropping. This week the temperatures will be in the mid-50s shortly after sunset. Sunset this week is around 6:40 PM and quickly getting earlier and earlier. The amount of lost sunlight will be slowing down as we approach the winter solstice in December.
If clear skies prevail, be on the lookout this week for the waning moon, Mercury and Venus, a ghost in the sky, and some outer planets overhead. I will also include a brief section on “weekly astrophysics” – gravitational tidal forces and a Jovian moon on the cusp of destruction.
The full moon was just this past Wednesday. So, if you are familiar with the lunar cycles, you know that it will take one week from the full moon to reach the third-quarter phase and another week to reach the new moon. This week you may notice the waning moon rising late at night or setting in the early morning after sunrise. Because the moon is rising so late into the night, this is not an ideal time to try observing the moon. It is much more “timely” and easy to observe when the moon is in its first-quarter phase, which will occur in roughly three weeks.
The Moon’s less familiar “Third Quarter” phase.
On Monday October 25, the innermost planet, Mercury, will be at its greatest western elongation. In several “What’s Up” posts before, I have mentioned “greatest eastern elongation” - so what’s the difference? Well during the greatest eastern elongation, the object appears highest in the sky, or at its farthest point east, around sunset. The greatest western elongation is just the opposite. The object will appear at its highest point, or farthest western point during sunrise times. Despite Mercury being at its farthest western position, it will still reach 12° above the eastern horizon before the sun rises and becomes too bright.
Mercury, as seen in the morning twilight early this week.
Following suit is Venus. Next Friday, October 29, Venus will be at its greatest eastern elongation. So look westwards just after sunset to see Venus at 17(degrees) about the SSW horizon. This is a great time to use binoculars or a telescope to observe this other inner planet. With strong enough magnification and careful observation, you may even make out the phase Venus is in – much like the moon!
Venus is the most prominent “star” in the evening sky until the end of the year.
Around this time of year, specifically around Halloween, there is a ghost in the night sky. This “ghost” is not very scary, but it is a reminder of the warmer summer months. Shortly after sunset, look west and try spotting the bright star, Arcturus. It will be between 10° and 15° above the western horizon just off the “arc” of the handle on the Big Dipper. Well, Arcturus is a ghost of the summer sun. Arcturus roughly marks the spot that the sun was in the sky at the time you are looking at back in June. So if you spot Arcturus at 7 PM, that is where the sun stood in June at 7 PM.
Arcturus marks the spot near where the sun was back on June 7.
Any time this week, peer south to find the bright outer planets of Saturn and Jupiter. They will both be crossing the meridian, or the line from south to north relative to your observing site, at 7:30 PM and 8:30 PM respectively. This will mark the highest points in the sky this time of year, so grab your binoculars or telescope and do some observing! Try to spot all four Galilean moons around Jupiter and the Cassini division in Saturn’s rings.
Jupiter and Saturn are still highlights of the evening sky, in the south.
Now onto some weekly astrophysics: Do you know what causes the tides? Many people who remember their high school or college science classes will tell you that the moon causes the tides, which is correct. In the same way that the moon’s gravity pulls on the Earth to cause the tides, Earth’s gravity pulls back on the moon. The moon doesn’t have any water that will slosh around, making the tides obvious, but the tides from Earth, cause the solid body of the moon itself to stretch out a few feet.
This effect happens on other planets and their moons, too. Io, one of Jupiter’s four largest “Galilean” moons is a fun example to look at. It orbits very close to the gas giant and undergoes extreme stretching by Jupiter’s gravity as it moves closer and further away from the planet in its orbit. Between the two extremes, Io stretches by around 200 feet! The kneading of the body of the moon heats Io up and helps it stay geologically active (it has lots of volcanoes) . Io is also near the Roche limit – the minimum distance from a large body before tidal forces will destroy a large orbiting satellite.
I am sure glad we do not have that problem with our moon!
Any large moon that gets too close to its parent will be ripped apart by tides. The point where the moon is no longer able to hold together against the tidal stretching is called The Roche Limit.
This week is not looking very promising for stargazing since the Ohio weather is giving nothing but clouds. We can still hope the forecast is wrong for later next week and may give some clear nights. If things clear up, grab a blanket or chair, a warm jacket, and your binoculars and telescope to explore the cosmos. Try spotting the waning moon, the greatest elongations of the two inner planets, two gas giants crossing the meridian, and a ghost of the summer. And think about how Io is nearly being ripped apart by Jupiter the next time you can see it through your telescope.
Clear Skies! (Hopefully)