(Sky Watch is a regular column written by Mike Shanahan, director of education and exhibits at the Bishop Museum in Honolulu. He can be reached at mikes@bishopmuseum.org.)

If the weather cooperates, we’ll see a spectacular total lunar eclipse from the Hawaiian Islands on the evening on Monday, Dec 20.

About 9:45 p.m. to 10:50 p.m. on that evening, the moon should have a dark, eerie red color. While we have lunar eclipses of some sort every six months, this is the first total eclipse of the moon in three years. In a total lunar eclipse, the entire moon goes into the deep inner shadow of the earth. The moon darkens dramatically, and often turns deep red.

Before going any further, I need to discuss this really important issue of timing. This total lunar eclipse occurs on the evening of Monday, Dec. 20 in Hawaii, not Dec. 21.

This is a classic example of the problems we encounter in Hawaii with the times for celestial events. Such timings are generally given in Universal Time. Universal time means, roughly, “the time it is in Greenwich, England,” just outside of London.

Greenwich, like the rest of England, is 10 hours ahead of Hawaii (at least when England is not on Daylight Saving Time). So when it’s 10 p.m. Dec. 20 in Hawaii (the peak of the lunar eclipse), it’s 8 a.m. Dec. 21 in Greenwich, England. In Universal Time this is written as “0800 hours,” since Universal Time, like military time, doesn’t use ‘AM’ and ‘PM.’

So, you might see information about this upcoming lunar eclipse as occurring “Dec. 21”; but keep in mind that it occurs Dec. 21 in Universal Time. This beautiful event will occur Dec. 20 in Hawaii, and if you go out and try to see the eclipse on the evening of the Dec. 21, you’ll be literally a day late.

Lunar eclipses occur when the earth comes in between the moon and sun. Lunar eclipses can only occur during full moons. Only then do we have a straight line between sun, earth and moon, with the earth in the middle. The earth blocks most of the sunlight and stops that light from hitting the moon.

If the earth had no atmosphere, the moon would turn black during a total lunar eclipse. However, we do have an atmosphere. As the sun’s light passes by earth on the way to the moon, earth itself blocks most of the light. In addition, the earth’s atmosphere blocks most of the sun’s light from passing through.

However, our atmosphere does allow the long wavelengths of red light to pass through the atmosphere and continue onwards to the moon. Thus, during lunar eclipses the moon does not turn black; it turns an eerie shade of dark red.

This deep-red effect is generally most noticeable during total lunar eclipses like the one Dec. 20. (During partial lunar eclipses, when only part of the moon is in the earth’s deep shadow, the reddish effect is not as striking.)

From the Hawaiian Islands, the lunar eclipse technically starts at 7:29 p.m. as the moon enters the faint outer shadow of the earth. However, you will not notice any darkening of the moon until at least 8:32 p.m. Hawaii Standard Time, when the moon begins to enter the deep inner shadow (umbra) of the earth. At that time, the moon will be about one-third of the way up in the eastern sky.

By 9:42 p.m., the moon will be entirely in the earth’s inner shadow, as the total phase of this lunar eclipse begins. At this time, the moon will be about halfway up in the eastern sky. By this time, the moon should be dramatically darker and redder than usual.

This total phase, with the moon completely within the earth’s umbra, will last till 10:52 p.m. By the time the total phase ends, the moon will be almost exactly overhead, in the very zenith of the sky.

As of 10:52 p.m. the moon will start to leave the umbra, and you will see the moon start to lighten. By midnight the moon will be entirely out of that dark inner shadow and will look like a regular full moon. Technically this eclipse ends at 1:03 p.m. Dec. 21 as the moon leaves the penumbra completely.

While it is never safe to look at any phase of a solar eclipse (except for the few brief, rare moments of a total solar eclipse), lunar eclipses are completely safe for viewing. All you really need to view this event is clear skies.

As noted above, the moon will be halfway up in the eastern sky when the total phase starts at 9:42 p.m., so you should not have any interference from hills or buildings.

Another great thing about this eclipse is that the moon will very high overhead, especially here in Hawaii. As mentioned above, lunar eclipses only happen during full moons. A full moon in December follows the same path across that the sun follows in June, sweeping to the very top of the sky.

This Dec. 20 lunar eclipse occurs less than one day before the winter solstice. Within 24 hours we will see the full moon, in full eclipse, climb to the top of the sky Dec. 20; and then see the sun barely make it halfway up the southern sky at its highest point Dec. 21.

While solar eclipses are visible only over a small portion of the earth, half of the planet sees a lunar eclipse. This Dec. 20-21 eclipse will be visible throughout the Hawaiian Islands and throughout the eastern half of Polynesia (Tahiti, Easter Island, and Marquesas). This eclipse will also be total over nearly all of North America.

More information at this always-excellent NASA eclipse site:
http://eclipse.gsfc.nasa.gov/eclipse.html

Winter Solstice

Winter officially starts Dec. 21 at 1:38 p.m. Hawaii Standard Time (23:38 UT on Dec. 21). This marks the shortest day of the year for the northern hemisphere and the longest day of the year for the southern hemisphere.

Here in Hawaii, the sun rises at 7:05 a.m. and sets at 5:55 p.m., still giving us nearly eleven hours of daylight. Perhaps more striking, the sun is much lower in the sky this time of year.

While the sun passes overhead in Hawaii in summertime, the highest it gets Dec. 21 is 44 degrees above the southern horizon at noon. That’s almost exactly halfway up in the south.

To get a sense of the sun’s path in summer, check out the moon’s path during the total eclipse on the night before the solstice, Dec. 20. As noted above, the full moon in December follows the same path that the sun does in June – way high overhead.

The seasonal changes that come with the solstice get much more striking as we get closer to the poles. In a place like Seattle, for comparison, the sun does not rise Dec. 21 till 7:55 a.m. and sets by 4:20 p.m., giving Seattleites less than 8.5 hours of daylight. And at noon, the sun in Seattle is a mere 22 degrees above the south horizon, less that a quarter of a way up in the sky.

Planets

Jupiter

At minus 2.5 magnitude, Jupiter is the brightest dot of light in our December 2010 evening sky. You should have no problem finding it in the evening sky this month; it’s the brightest thing except for the moon in the evening sky.

Look for Jupiter high in the sky around 7 p.m. at the start of December. It will then work its way down the western sky, setting just after 1 a.m. in early December. By the end of December, Jupiter is about two-thirds of the way up in the southwest when it gets dark at 7 p.m., and sets at 11:30 p.m. Look for the first quarter moon next to Jupiter on Dec. 13.

Venus

Venus is now a blazing morning star. It rises in the east at 4 a.m. at the start of the month and about 3:45 a.m. at the end of the month. It’s about 1/3 of the way up in the east by daybreak.

At minus 4.9 magnitude, it outshines everything else and is impossible to miss (unless clouds get in the way). Look for the waning crescent moon next to Venus on Dec. 2 and again Dec. 31.

Saturn

Saturn has also popped into the morning sky. At the start of December, Saturn rises just before 3 a.m. and is halfway up the sky as day breaks. By the end of the month, Saturn rises at 1 a.m. and due south as day breaks. The planet shines at 0.8 magnitude. Look for the waning crescent moon next to Saturn on Dec. 1, Dec. 28 and Dec. 29.

Mars and Mercury hug the twilight

Mars finally leaves our twilight sky in December. For much of the month it is still there, just above the western horizon around 6:15 p.m., but so faint (first magnitude) that it will be really hard to spot. By Dec. 31, it’s gone completely.

Mercury also hugs the western horizon around 6:30pm for the first week of December. Look for it about six degrees above the western horizon (two fingers, held at arm’s length). At minus 0.36 magnitude, it is much brighter than Mars and you should have a better time spotting it.

On Dec. 7, you can use the slender crescent moon as a guide; Mercury will be the bright dot just below the moon on that night.

Stars

The December Sky Map

The map for December 2010 is good for 9 p.m. at the start of November, 8 p.m. in the middle of the month, and 7 p.m. at the end of November. As always, hold this map over your head or the four directions will never line up!

We’ll start in the southern part of the map and then work our way north.

Of special note, the familiar constellations of winter have returned! Look for Orion the Hunter rising in the east, with its familiar hour-glass shape, distinctive belt of three stars, and brilliant stars Rigel and Betelgeuse.

Other winter constellations in the low eastern sky include Auriga the Charioteer, with its brilliant yellow star Capella; the Gemini, marked by the twin bright stars Castor and Pollux; and Taurus the Bull, with the reddish Aldebaran marking its eye.

The brightest object on the map is Jupiter, marked on this map by a large round dot in the southern portion of the sky. Jupiter should “pop” more than ever, since it’s surrounded by very faint constellations. These include Pisces the Fish. Jupiter is just below the portion of Pisces called “the circlet.”

However, don’t expect to see the circlet or any other part of Pisces unless you are in a really dark sky. It’s a very faint constellation, made up mainly of 4th and 5th magnitude stars (a 5th magnitude star is about 40 times dimmer than a 1st magnitude star). To the right of Pisces are two other very faint zodiac signs, Aquarius (the water bearer) and Capricornus (the sea goat).

There are two bright stars in the south. You should have no problem finding the star Fomalhaut, which is south of Jupiter.

Fomalhaut is a first magnitude star in the constellation of Piscis Austrinis, the Southern Fish. You won’t be able to pick out the other faint stars (4th and 5th magnitude) in the constellation from city skies.

Also, very low in the south, look for Achernar, the bright star that marks one end of Eridanus the River. The River starts with Achernar and wends its way to the east, eventually ending at the feet of Orion the Hunter.

Below Fomalhaut is the constellation of Grus the Crane. The most distinctive part of Grus: two second magnitude stars, Beta Gruis and Al Nair, that pop out nicely against the background of one of the dimmest parts of the sky.

Al Nair is the one on your right as you face south, and Beta Gruis is on the left. While they don’t jump out on our map, they are distinctive in the “real” sky. Each of these stars is as bright as any of the stars in the Big Dipper.

Very, very low in the west you can still see the summer triangle, even though December is your last chance. This triangle is made up of three bright stars from three different constellations.

This includes Vega, the brightest star in the triangle, in Lyra the Harp; Altair, in Aquila the Eagle; and Deneb, the dimmest of the three bright stars, in Cygnus the Swan. We’ve shown both the individual constellations and the triangle on our map.

Looking north, the constellation of Cassiopeia if high overhead. Cassiopeia is made up of five stars that form a squished “W” shape. The three middle stars of Cassiopeia’s “W” form an arrowhead that points roughly to the North Star.

December is still a good time to see all of the constellations that tell the story of Cassiopeia, Andromeda and Perseus. There are six different constellations that play a role in this story, and all of them are visible on our map.

Cassiopeia ruled Ethiopia with her husband, King Cepheus. Cepheus is also there in the sky tonight. His constellation is high up in the northwest and he looks like an upside-down house, with the roof pointing to the horizon.

Cassiopeia was famously vain. To punish her, the gods created a sea monster that then attacked Ethiopia. In Greek mythology the sea monster that ravages Ethiopia is identified with the constellation of Cetus the Whale, which is due south on the November sky map.

To appease the gods, Cassiopeia and Cepheus were told the sacrifice their daughter Andromeda to the monster. Andromeda’s constellation is visible on our map as well, as a strand of stars connected to Pegasus.

Andromeda was saved in the nick of time by a hero named Perseus. Perseus’ constellation is high in the northeast on our star map. The constellation looks like a night cap.

Just before saving Andromeda, Perseus had slain Medusa, the famous snake-haired monster whose look could turn you to stone. From the blood of Medusa sprang the winged horse Pegasus, which is the square-like constellation overhead on our map. Perseus takes Medusa’s head, puts it in a bag, and goes winging for home.

Perseus flew over Ethiopia on the way home; saw Andromeda tied to the rocks and about to be eaten by the sea monster; and turned the monster to stone with the dead head of Medusa.

Stars that aren’t on the map:

The Big Dipper is missing. The Dipper will be entirely up by 2 a.m. at the start of December; midnight by the end of the month.

The Southern Cross returns to our morning sky as it does every December. It rises in the southeast at 6 a.m. in early December, when you have only a few minutes to catch it before daybreak. By the 15th it’s up by 5 a.m. and you have a hour till daybreak. By the end of December, the Cross rises at 4 a.m. and you have two hours prior to dawn to spot it.

Look very low in the south for the Southern Cross; you need to make sure there are no buildings or trees blocking your south horizon. Its bottom star, Acrux, only gets about six degrees above the horizon at its highest point, or the width of two fingers at arms length.

Moon phases

New Moon: Dec. 5

First Quarter: Dec. 13

Full Moon: Dec. 20

Third Quarter: Dec. 27

— Find out more:
Bishop Museum Planetarium: www.bishopmuseum.org/exhibits/planetarium/planetarium.html
Hawaiian Astronomical Society: www.hawastsoc.org/