Caoimhín's Content

Another month, another preview of upcoming astronomical phenomena! December is a great month for astronomy thanks to the long nights, including the longest night of the year! There is also a meteor shower this month, the Geminids in the middle of the month, nice and close to the New Moon, so the sky should be nice and dark. The Full Moon this month will fall on the 27th, a bit after the solstice and just before the New Year.

Firstly, let’s cover the planets. Jupiter and Saturn are still both up in the evening, with Venus popping up in the morning. Uranus is still right in between the Pleiades and Jupiter if you have a telescope. The change this month is Mercury! Mercury is at its Greatest Eastern Elongation, it is at it’s furthest out from the Sun in the evening. Despite being the Eastern Elongation, we need to look into the West as the Sun sets to have a chance of seeing it. Unfortunately it is only a chance, and really not a very good one. Thanks to the angle at which the Sun is setting, Mercury will be incredibly close to the horizon, and mostly obscured by the glow of sunset. You might be able to catch it with a telescope, but with the amount of light in the sky it’s important to be careful.

Speaking of sunset, let’s push back to sunrise. The shortest day of the year starts with a nice long sunrise thanks to the angle of the Sun rising. We begin to see a glow of sunrise as early as half 7, but the Sun won’t really get above the horizon until practically 9. Even then, it only appears to be fully above the horizon thanks to our atmosphere refracting the light. The same thing happens in the evening, with the Sun appearing to rest on the horizon close to three, before sky finally darkens at just around 4:30. The shortest day of the year lives up to its name, being bright for only about 8 hours, with the night taking up the rest of the 24 hours of the day.

The length of the day isn’t the only difference, the Sun is also much lower in the sky. Both of these effects are due to the tilt of the Earth. The Sun is staying in the same place, on the ecliptic. With the Northern Hemisphere tilted back, away from the Sun, we see a much smaller segment of the ecliptic above the horizon, keeping the day short, the Sun low in the sky, and the sunset and sunrise long. If we were in midsummer, of course the sun would appear much higher, with a lot more of the ecliptic above the horizon. This keeps the day long, and at sunset and sunrise the intermediate dusk and dawn stages are much shorter. This also means that planets near sunset and sunrise will be more above the Sun, rather than alongside it, which makes them a little easier to observe.

The apparent motion of the Sun up and down throughout the year is known anathema, and the Moon does the opposite. The Full Moon will appear much higher during the winter, lower in the summer. Due to the lunar cycle, this can be a little trickier to show, but it is noticeable if you compare the closest Full Moon to the winter solstice to the one nearest the summer solstice. These don’t always line up, though they can, and the Full Moon this year is only a few days after the solstice. The solstice falls on the 22nd while the Full Moon is the 27th.

Luckily, the Moon won’t be full on the night of the 14th, which is the night of the peak of the Geminid Meteor shower. There is a previous video and article on this website that deals with meteor showers in depth (as well as videos about the Moon, planets and ecliptic for that matter). Here, a few of the upcoming meteor showers are referenced, just to show that we often have many running at the same time. many are only minor, adding a couple of meteors an hour to the background rate. Although we will have many concurrent meteor showers, it is the Geminids that will make a true difference, with up to 120 meteors passing the zenith every hour. Although the Leonids can historically be much more impressive, the Geminids could equal or surpass the more normal Leonid rate that we saw in November.

For a quick look ahead I believe that is long enough. Many parts of this video will have their own references dedicated to them in the future, especially the solstices and the apparent motion of the Sun and the Moon throughout the year. In fact, we might take a more detailed look at the solstice’s closer to the upcoming winter solstice in a few weeks! I hope you will join me back here for that.