The Sky for December 2018

This is the start of a new feature of my blog, whereby I share my astronomical knowledge, accumulated through 45 years, plus about 20 of observational astronomy.

In the sky this month, there are a few things to catch your attention. Let's focus on those.

Earth's Shadow

This is often noticed in December, when sunrise and sunset often coincide with our morning or evening commute. It happens shortly before sunrise, and shortly after sunset, on clear days.

What you notice is, near the anti-sunrise/set point (opposite from where the sun is about to rise or has just set), a darker slice of atmosphere hugging the horizon. As sunrise approaches, it seems to concentrate right above the horizon. As more time passes after sunset, the band rises higher and dilutes.

What you're seeing is the Earth's shadow, cutting up through our atmosphere. Very near to sunrise or sunset, this is a very slight angle, so you're looking 'straight down' the shadow; and thus it seems quite dark. Further away from either time, The angle is less shallow (the shadow is angled more), and the effect fades.

Venus

In December mornings, before sunrise, a bright, star-like object can be seen rising into the sky in the Southeast. It's brighter than virtually anything else in the sky, save for the Sun and the Moon, and shines with a steady, untwinkling light.

What you're looking at is Venus, Often referred to as Earth's sister planet. This is because both its diameter is just a few-hundred kilometres smaller than Earth's, and also because of its proximity to our own orbit around the sun (Venus, average distance about 108-million km; Earth, average distance about 149-million km).

On closer examination, though, the claim of sisterhood falls apart, A thick, carbon-dioxide atmosphere cloaks the planet, smothering the surface under its crushing weight. Surface temperatures are hot enough to melt lead. What's more, Venus actually rotates retrograde (against its orbit, so to speak), and its day (243 earth days) is actually longer than its year (277 days).

The Soviets soft-landed a spacecraft on Venus, in the mid-1970s; surface photos were recovered.

Incidentally, Venus, like most of the planets, doesn't twinkle. That's because, while tiny, the disks they display tend to be larger than the turbulence cells in the upper atmosphere that cause twinkling; thus, the cells mostly cancel each other out, and no twinkling.

Sirius

Rising in the Southeast late in the evening is Sirius, once known as the Dog Star. Sirius gets its nickname by virtue of being the brightest star in the constellation Canis Major (the Great Dog). It's a bit larger and brighter than the sun, and it has a companion--Sirius B, a white dwarf (a star that has used up all of its nuclear fuel and, because of its relatively small mass, died a quiet death).

I mention Sirius B because the Dogon Tribe, in Africa, holds a festival once every so often to commemorate Sirius B's orbit. The catch: Sirius B is much too faint, and too close to Sirius A, to be seen by the naked eye.

Canis Major, Sirius' home constellation, is one of several companions to great Orion, the Hunter. (Two others are Canis Minor (the Lesser Dog), with its bright star Procyon; and Taurus (the Bull), and its bright red-giant star, Antares.

Sirius twinkles madly, most nights, and seems to display most of the colours of the rainbow. That's because it is, through atmospheric refraction. Other stars do it, too—they're just not bright enough for you to notice.

Orion

In my childhood, I was often stopped in my tracks by the sight of a large X in the Spring sky. After not too many years, I learned that it was Orion, the Hunter. You've probably seen it; a large X, with four bright stars prominently marking the corners, and three fainter stars in the middle. Well, that's Orion. The three stars in the middle are called the Belt. Three further stars 'hang down' from the belt; this is the Dagger.

Now, a curious thing is the Dagger. In a clear, country sky, you can notice with the naked eye that one of the stars in the Dagger is fuzzy. And, indeed, binocular or telescopic aid will reveal that you are looking at a huge, interstellar gas cloud.

What you are looking at is the Great Orion Nebula, the subject of so many telescopic portraits over the years. You've doubtless seen Hubble images of the many gas/dust 'coccoons' inside which new stars and planets are forming.

Those are the major sights for which to keep an eye out over the next month. Some other events (in brief): earliest sunset (mid-latitude) Dec. 11; Winter, or Southern, Solstice Dec. 20.

That's it for this month. If you don't have a good pair of binoculars, do invest.

-Bill