The International Space Station (ISS) finishes its set of evening passes in early December, takes a break, then resumes matinee appearances around mid-month. The first of these occurs around 07:12 on Dec. 15 when the ISS will rise in the southwest and move across the southern horizon just before passing less than one degree (one thumbnail width at arm’s length) below brilliant Venus at 07:15. Although this is 45 minutes before sunrise, they’re both bright enough to see in the morning twilight with ease. The Heavens-Above website has the best info on these. In fact, the ISS repeats this type of pass every morning for the next week, rising in the south, passing above or below Venus in the southeast and setting in the east. This is all because our rotation axis is tilted 23.5 degrees from our orbital plane and the ISS orbit is tilted 51 degrees from our rotation axis. Yeah, I still struggle trying to visualize this.
The Geminid meteor shower occurs every December, reaching its predicted peak Dec. 14. The moon will be just past new, so setting right after sunset, and the best observing is likely to be after midnight that morning with a chance of seeing some the day before and after. The radiant will be beside the bright star Castor in Gemini (left and above Orion). The Geminid shower is one of the more populous we see – a Zenithal Hourly Rate of 120, for example. The typical meteors are white and bright and not particularly fast, compared to Perseids or Leonids, for example. As with all showers, optical aids are not needed; just find a warm comfortable place to lie down under an open sky and look up. While the meteors will appear to radiate from Castor, they can appear anywhere in the sky.
Although the Geminids is one of the higher ranked showers, the really weird thing is that the source of all the meteoric material does not appear to be a comet – a “dirty snowball” whose icy surface vaporizes with each close pass to the sun – but rather a rock, the asteroid 3200 Phaethon. It was discovered in 1983 but its orbital calculation in 1985 shows it to have an orbit consistent with that of the Geminids but more like a comet than an asteroid. While asteroids commonly orbit in the gap between Mars and Jupiter, 3200 Phaethon has a highly elliptical orbit with a perihelion of only 13 million miles from the sun (inside Mercury) and an aphelion of 222 million miles (out past Mars). I gather it was so named because Phaethon was the son of Apollo, the Greek god of the sun. Its orbit is the most elliptical of any named asteroid and its orbital period is about 1.4 years. Since it’s not overly large – about five kilometres in size – its remarkably close approach to the sun and small gravity may mean that heating stresses crack its surface and solar wind and radiation pressure can push small fragments into space. Think of it as a large rock with a scalp condition. The solar heating effects are NOT trivial, by the way; SPF 200,000 won’t help when the sunshine is enough to melt aluminum.
Jupiter is still big and bright, visible basically all night, since it’s just past opposition. As I’ve mentioned before, a stable pair of binoculars or a simple telescope is enough to allow you to see its four big moons, known as the Galilean moons: Io, Europa, Ganymede and Callisto. Discovery of these moons has been credited to Galileo and their names later selected from some of the Greek mythological lovers of Zeus, the King of the Gods. Apparently, if you’re the King of the Gods, you can get away with a lot and don’t need to be politically correct; it would seem though that his wife, Hera, was unavailable for comment at the time. These moons will be widespread on the 5th and 15th and grouped extremely close to Jupiter on the 9th in the early morning before Jupiter sets. It’s a show that never gets old. RASC members can refer to the Observer’s Handbook for this information but, honestly, the best (and free) resource is to download and install Stellarium. I’ve had it on my computer running Windows 10 for years but I gather there are other operating systems that may appeal to the more technically adept. I cannot recommend this software highly enough!
The Sunshine Coast Astronomy Club monthly meeting will be at the Sechelt Library at 7 p.m., Dec. 8, at which The Sky This Month will be presented. Please check the club website at: sunshinecoastastronomy.wordpress.com/ for the subject of the guest lecture. Non-member admission will be by donation and refreshments may include coffee and cookies.