Princess Andromeda, the Moon masks Aldebaran, and Mercury Attends the Pre-dawn Planet Party!

Star Walk
8 min readJan 3, 2018


Above: This image of the Andromeda Galaxy and its two smaller companions was taken by Stephen McKinney of Toronto. The distance from the lower left to the upper right spans six full moon diameters in the sky! In a large telescope, the dark dust lanes in the spiral arms can be seen.

Princess Andromeda’s Treasure

A few weeks ago, we started touring the constellations involved in the story of Perseus and Andromeda. This week, I’ll tell you more about the princess herself. I’ll post a sky map here.

Andromeda was the beautiful daughter of Queen Cassiopeia and King Cepheus of Ancient Ethiopia. Through no fault of her own, the princess became the centre of a hair-raising story. After her mother angered the Nerieds (Sea Nymphs) by boasting of Andromeda’s unrivaled beauty, the god of the sea Poseidon (aka Neptune) sent Cetus (the Sea-monster) to ravage Ethiopia’s coast. An oracle told King Cepheus that his only solution was to sacrifice Andromeda to Cetus. So she was chained to rocks by the sea. Just as Cetus was about to take Andromeda, the hero Perseus happened to be flying by. She must have been beautiful indeed, because he fell instantly in love with her, slew the beast, and cut her free from her chains. After a few more adventures, including one where Perseus accidently killed his evil grandfather to fulfill a prophecy, the couple sailed to Argonis, where they raised a family of many children. Hercules is descended from them.

Andromeda is one of the original classical constellations, and is 19th largest by area (out of 88). Also known as the Chained Lady, she is depicted as laying prone with her body and legs extending to the east (left) using two chains of stars that diverge towards her feet. Another set of dimmer stars extends upwards, near the Milky Way. These represent her chains. A number of ancient cultures saw a woman’s figure in those stars. The Chinese also saw legs, but use a different arrangement of stars.

Andromeda is visible throughout the fall and winter, but right now she is directly overhead in early evening. She is located adjacent to Pegasus on his eastern (left) side. Her brightest star, named Alpheratz “the Horse’s Shoulder”, marks her head, and is also serves as “third base” of Pegasus’ great square. Pisces (the Fishes) sits below her, as does Triangulum (the Triangle). Her mother Cassiopeia is above her, and Perseus lies to the east, below her feet. but he’s slightly to the upper left in the sky.

Let’s tour the best of Andromeda. Start at bright Alpheratz. It’s a hot blue-white supergiant star located only 97 light-years away from us. The spectrum of this star’s light indicates that it is highly enriched in the metal Mercury. To see the rest of her, look for two slightly bent, roughly horizontal, lines of three modest stars each extending off to the northeast (left). The two lines are longer than the great square’s diagonal, and the lower set is brighter than the upper set. They diverge as you move away from Alpheratz, the way her gown would spread out towards her feet.

Tracing the lower line of three stars from Alpheratz, the first star we come to is reddish Delta Andromedae, roughly a palm’s width left from Alpheratz. The next star, another palm width left, is a bright reddish star called Mirach. This is a cool red giant star located 200 light-years from Earth that produces 1,900 times more light than our sun! Mirach happens to have a distant elliptical galaxy beside it called Mirach’s Ghost, but you need a large telescope to see that fuzzy patch. (Make note of Mirach — we’ll return to it later.)

Continuing left by a fist’s diameter, we reach one of Andromeda’s feet — the star bright Almach “desert lynx”. This very beautiful golden and sapphire pair of stars easily is seen in a small telescope. Almach’s stars differ in brightness as well as colour.

Now, back to Mirach. Sitting about four finger widths above Mirach is a dimmer white star designated Mu (μ) Andromedae. (It’s actually the centre star of Andromeda’s upper chain.) A few finger widths above that star is another yet dimmer star. Look just above that one for a large fuzzy patch. That’s Messier 31 (or M31 for short), better known as the Andromeda Galaxy — one of the sky’s best sights! This massive twin to our Milky Way galaxy spans three full moon diameters across the sky. It has a bright core and dim oval halo that is oriented east-west (left-right). It’s easy to see with binoculars. Using a good sized telescope, more is revealed — including two smaller galaxies sitting just above and below it. One of these is closer than M31 — the other is farther.

Under dark skies, you should be able to glimpse the galaxy using unaided eyes only. At about 2.5 million light years away, it’s one of the farthest objects visible to unaided human eyes! We’re going to collide with it in about 5 billion years, but that’s another story!

The Moon and Planets

The moon repeats its cycle of lunar phases every 29.5 days, and this is the best week to view it in your new binoculars or telescope. Whenever the moon is partially lit, the region along the terminator, the boundary that separates the dark and sunlit sides, is especially beautiful when viewed close up. This is because the sun is rising for locations along the terminator, and casting long black shadows between brightly lit crater rims and mountain peaks. Sunrise is much slower on the moon than Earth because the moon rotates only once per month (the same time it takes to orbit the Earth), so it’s “day” is very long.

Each night this week, new vistas on the moon will be dramatically illuminated. The First Quarter phase, when the moon is half illuminated on its eastern (our western) side, occurs on Tuesday at 4:20 am Eastern time.

The moon’s orbit is tilted by about 5° from the plane of the Solar System — so it bobs up and down across the plane while it circles Earth, frequently passing near to, and sometimes in front of, the planets. This bobbing also makes solar and lunar eclipses infrequent, since the moon is usually higher or lower than the sun at new moon — and misses it completely.

I’m mentioning the moon’s motion here because tonight (Sunday), the moon will be sitting 5° (a bit less than a palm’s width) to the left of tiny Neptune, and on Wednesday night, the moon will be 5° below Uranus. That 5° separation won’t help you locate the planets that easily, but it’ll give you an idea of where they are in the sky after the moon passes on.

Uranus and Neptune are the only planets left in the evening sky, setting about 2 am and 10:30 pm local time respectively. Blue-green Uranus is midway between the two chains of stars that form the dim constellation of Pisces (the Fishes), about two finger widths higher than the two visible stars Omicron (o) and Mu (μ) Piscium that bracket it. Tiny blue Neptune, which can only be observed in a telescope, is about half a finger’s width below the medium-bright star Hydor in Aquarius (the Water-Bearer).

For the second time this month, the moon returns to cross the face of Taurus (the Bull) on Saturday evening. This time, for observers in most of North America, Greenland, and parts of Europe and Russia, the moon will pass in front of (or occult) Taurus’ brightest star Aldebaran. In the GTA, the star will wink out behind the moon’s dark leading limb at 6:20:30 pm Eastern time, and reappear at the opposite lit limb at 7:19 pm. Times vary slightly by region.

You can watch the event in binoculars or, better yet, a small telescope. Start watching a few minutes ahead of time, while Aldebaran is still nicely separated from the moon. Remember that the moon is closer to the star than you think because its leading edge is darkened. You can watch as the moon closes the gap — and then the star will suddenly disappear. (If the moon had an atmosphere, the star would gradually fade out!) Feel free to zoom in with your telescope for that part. If you’re not sure where Aldebaran will pop out from behind the opposite side of the moon, use a lower power eyepiece that shows all or most of the moon’s bright right-hand edge. As before, start looking a couple of minutes beforehand. And don’t look away — it re-appears suddenly!

The planet party is in the pre-dawn sky! If you’re an early riser, you’ve probably been seeing a very bright object shining in the eastern dawn sky. That’s Jupiter, which rises just before 4 am local time this week. It’s well above the southeastern horizon by dawn. It might remind you of the Christmas star shining in the east! At the same time, have a look at the bright star sitting only a finger’s width to the right of Jupiter. That’s the double star Zubenelgenubi in Libra (the Scales). Try your binoculars on it. That unusual name comes from the Arabic phrase for “southern claw”, because the star used to be part of Scorpius (the Scorpion).

Much dimmer, and red-tinted, Mars rises about 30 minutes before Jupiter. Look for it sitting to Jupiter’s upper right. On Christmas morning, Mars will be about a palm’s width from Jupiter. But over the course of this week, Mars’ eastward orbital motion will carry it towards Jupiter, so that the separation between them will shrink to half of that next weekend. Keep an eye on them. The two planets will “kiss” next week!

This is an absolutely terrific week to see Mercury with your unaided eyes or binoculars! The normally elusive planet is climbing the pre-dawn sky this week and then descends next week. It’s going to be quite easy to see between about 6:30 and 7 am local time. It will be the medium bright “star” sitting low in the east-southeastern sky. I’ll post sky charts for all the planets here.


Iridium flares are glints of sunlight off of the flat reflecting sides of one of the satellites that comprise the Iridium pager and sat-phone network. The flares occur before dawn and after dusk, when the satellite passing overhead is still illuminated by the Sun, which is below the horizon for observers on the ground. The duration and brightness depend on the angles between the observer, the satellite, and the Sun. For even more info about Iridium Flares and the space station, see an article that I wrote here. Using an accurate clock, go outside a few minutes ahead and look in the direction indicated. You should first see the dim Iridium satellite moving quickly across the sky, and then it will rapidly brighten for 3 to 8 seconds, and fade out. Truly spectacular! The more negative the Magnitude number, the brighter. The larger the Alt. number, the higher up it is! (The horizon is 0°, and 90° is straight up, so 55° is a bit higher than halfway between the horizon and zenith.) These data are adapted from To get your own schedule, enter your location in their website.

The ISS (International Space Station) is also visible at times, gliding silently overhead. If you enter your location in the website, you will get a list of them for wherever you might be.)

Keep looking up to enjoy the sky! Astronomy Skylights for this week (from December 24th, 2017) by Chris Vaughan.

Happy Holidays, everyone! Or, as we astronomers say, “Have a Happy Solstice and a Merry Perihelion!”



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