Delving into the Dippers

In the northern sky at this time of year, the Big Dipper stands upright on the tip of its handle in mid-evening. If you turn and face north, you’ll find it centred only a few fist widths above the horizon, a little bit east (left) of north. After midnight, the dipper climbs to sit nearly overhead.

The Big Dipper is not actually a constellation — it’s an asterism! In the UK, they call this star pattern The Plough. The ancient Chinese, who also saw a dipper in these stars, used the orientation of the Big Dipper to mark the seasons — when it stood upright at dusk, spring was around the corner.

The dipper’s seven bright stars are only a portion of a large constellation called Ursa Major (the Big Bear). Fainter stars located above and to the right of the dipper form the bear’s head and neck, and its front and rear legs. The bowl of the dipper looks a bit like a saddle on the bear’s back. The dipper’s handle doubles as the bear’s tail. But, wait a minute — real bears have stubby tails! Well, legends say the bear was swung into the night sky by its tail — stretching it!

Above: In February annually, the Big Dipper, an asterism made from the seven brightest stars of Ursa Major, sits upright on its handle over the northern horizon. The dimmer stars of the Little Dipper, including Polaris, sit due north, opposite its larger counterpart. The sky is shown for 9 pm local time in mid-February. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.

Using the Big Dipper’s orientation during February evenings, the highest two stars mark the outer rim of the dipper’s bowl. Dubhe “doo-bee”, on the left, is a medium-hot, orange giant star located about 124 light-years from Earth. Hot, white Merak, to its right, is only 80 light-years from Earth. In the sky, the two stars are separated by 5.3°. Stretch your hand out in front of you and compare their separation to your palm’s width or various finger combinations. The name Dubhe is a shortened form of an Arabic expression that means “back of the Greater Bear”, while Merak means “the flank of the Greater Bear”.

Moving downwards, the star where the handle meets the bowl is named Megrez, which means “the root of the Greater Bear’s tail”. This star is the dimmest of the seven. It’s a medium-hot, white star located 81 light-years from us. The star at the lower right corner of the bowl is Phecda, from “thigh of the Greater Bear”. (Hmm — I think there’s a pattern here.) It, too, is a white star about 80 light-years away.

Astronomers believe that five of the stars in the dipper formed together about 300 million years ago, and are still travelling through space together as siblings — hence their similar distances from Earth. The three stars in the dipper’s handle (from highest to lowest) are Alioth, Mizar, and Alkaid. Alioth is another of the “siblings”, while Alkaid is a slightly cooler, warmer coloured star located 104 light-years from Earth.

(Above: The sky in Ursa Major is littered with distant galaxies (coloured ovals), including the closer and brighter Pinwheel Galaxy and Whirlpool Galaxy which flank the handle stars. For best results, wait until late evening to view them, when the constellation has risen nearly overhead.)

Mizar, the hot, white star at the bend in the handle, has a dim companion star named Alcor sitting just to the lower left of it. Most people can see Alcor without magnification. In ancient times, it was used to judge the eyesight of soldiers. In some cultures, Mizar and Alcor are called “The Horse and Rider”. In a small telescope, Mizar separates into a lovely double star.

Ursa Major is located well away from the obscuring stars, gas, and dust of the Milky Way, allowing us to see into the distant universe in that part of the sky. The entire constellation is loaded with galaxies, including the spectacular Pinwheel and Whirlpool Galaxies.

If you draw an imaginary line from Merak though Dubhe and extend it to the left (north), the next major star you’ll encounter is Polaris, the North Star. Using your new sky angle measuring skills, you should come up with 29° from Dubhe to Polaris.

(Above: Messier 51, the Whirlpool Galaxy, sits 3.5° to the right of the star Alkaid. Image by Ian Wheelband in 2016. The link to his Smugmug gallery is here.)

Polaris marks the tip of the handle of the Little Dipper. At this time of year at around 8 pm local time, the rest of the Little Dipper hangs downwards from Polaris, and curves strongly towards the Big Dipper. The two bowls pour into one another, flanking the tail stars of Draco (the Dragon), a large constellation that wraps around the Little Dipper.

The asterism of the Little Dipper also makes up the constellation of Ursa Minor (the Little Bear). This little cub’s tail is stretched even longer than its larger parent. Other cultures have interpreted those stars as a dog or a wolf, which explains the lengthy tail much better!

The star at the outer edge of the Little Dipper’s bowl (and closest to the Big Dipper) is slightly dimmer than Polaris. This medium-cool, reddish star is named Kochab. The other five stars of the constellation may be too dim to see from the city, but binoculars will reveal them.

Polaris is located about one finger’s width from the celestial pole — the point in space directly above the North Pole on Earth. As Earth rotates on its axis, all the stars rotate counter-clockwise (west to east) around Polaris. Polaris’ fixed location makes it especially useful for navigation — at least from the northern hemisphere, where it never sets. The point on the horizon directly below it is due north, and the star’s angle above the horizon gives you your latitude.

Contrary to popular belief, Polaris is a very modest looking star, only 48th brightest in the night sky. Viewed through binoculars or a small telescope, Polaris also serves as the diamond in a crooked ring of dim stars. Coincidently, the ring is a finger’s width across!

The Moon and Planets

The moon graces the evening sky this week, shining prettily as a slim crescent over the western horizon after sunset for the next few evenings. Every night, it will wax a little fuller and remain in view later. This week is the best week of the lunar month to view the moon in binoculars or a telescope. The craters and mountains along the terminator line that divides the bright and dark hemispheres of the moon will be resplendent with dramatic shadows cast by steeply slanted sunlight.

Above: In the southern sky on the evening of Friday, February 23, the First Quarter moon will traverse the stars making up Taurus the Bull’s face, shown here at 8 pm local time. Observers elsewhere on Earth will see the moon occult the star Aldebaran. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.

On Friday, the moon will reach its First Quarter phase, when the relative positions of Earth, sun, and the moon cause us to see it half illuminated — on the western (right-hand) side. First quarter moons rise around noon and set around midnight, so they are visible starting in the afternoon hours. Also on Friday evening, the moon will be positioned a few degrees to the upper left of red-tinted Aldebaran, the brightest star in Taurus (the Bull). Hours before, skywatchers with telescopes in Bermuda, northeastern North America, Greenland, and most of Europe can see the moon’s orbit carry it across Aldebaran in daylight. Farther east, in Svalbard, most of Russia, Kazakhstan, western Mongolia, and northwestern China, the event will occur in a dark sky. Next weekend, the waxing gibbous moon will swing through the club of Orion and the legs of Gemini (the Twins).

Bright Venus continues to emerge from the western evening twilight this week. You can look for it very low above the western horizon after sunset, particularly between about 6 and 6:30 pm local time. Don’t fret if you can’t find it — Venus is just beginning a long stay in the evening sky that will kick into high gear this spring.

Above: Venus continues its long climb away form the sun in the western evening sky this week, while speedy Mercury races to catch it. The sky is shown at 6:20 pm local time on February 25, 2018. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.

Elusive Mercury is also returning to visibility later this week. It will climb towards Venus, reaching a point about four finger widths below Venus on the weekend. Mercury’s return kicks off a very good evening appearance for northern hemisphere observers during the next several weeks.

The pre-dawn sky is where you’ll find the rest of the bright planets, and an asteroid. Extremely bright Jupiter rises first, just after midnight local time. By 6:30 am local time, it reaches an elevation of about three fist diameters above the southern horizon. There’s no mistaking it. Dimmer, reddish Mars, which rises about 2:45 am local time, is sitting two fist diameters to the lower left of Jupiter this week, but it’s slowly moving downwards and left (eastwards) towards Saturn. The medium-bright, yellowish ringed planet rises about 4:30 am local time, arriving only about 1.5 fist diameters above the southeastern horizon by dawn.

Above: The southeastern pre-dawn sky this week features very bright Jupiter, then dimmer reddish Mars, and finally medium-bright yellowish Saturn — together defining the Ecliptic, the approximate plane of our Solar System in the sky. The sky is shown for 6:15 am local time, but Jupiter will be visible until sunrise. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.

Asteroid/minor planet fans can still hunt down Vesta and the dwarf planet Ceres this week. Vesta is located only 5.5° (a palm’s width) to the upper left of Mars. Brighter Ceres is in Cancer (the Crab) in the eastern evening sky. Since the stars of Cancer are so dim, look for Ceres roughly midway between the bright stars Algenubi, which is the nose of Leo (the Lion) and Pollux, the lower of the twin stars in Gemini. Ceres remains visible all night long. You’ll need binoculars or small telescopes to see these two objects. I posted a sky chart for Ceres here.

Astronomy Skylights for this week (from February 4th, 2018) by Chris Vaughan. Keep looking up to enjoy the sky!