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The Sky Guy

Night sky still worth a look

Posted: Sunday, April 01, 2007

 

  Graphic courtesy of Andy Veh

Graphic courtesy of Andy Veh

Days are getting longer and nights are getting shorter. We’re also leaving the freezing cold behind. As we all love winter’s benefits, we surely regret that our beloved starry skies will take a back seat soon.

But at least we get some nice views for a goodbye. With daylight-Saving time having started, it doesn’t get dark until 10 p.m. or even later and therefore the diagram shows the western sky around 11 p.m. toward the end of April. This late in the season, many of the brilliant stars of winter are either gone or hard to detect. Therefore look early in the month in the west for Sirius and Orion with Betelgeuse and Rigel, Aldebaran and the Pleiades. Still visible all month are the twins Castor and Pollux in the middle of the diagram, Procyon below and left of middle, Capella in its pentagon of Auriga on the right and, having shifted from east to west throughout late winter, are Leo with Regulus and Saturn nearby. The latter is actually of virtually the same brightness as these bright stars and thus blends in pretty good, so you would need this diagram or a starfinder to locate Saturn.

Visible after sunset for about two hours is very bright Venus in the West and since it will stay there all summer, it will probably be the only heavenly object -- aside from our moon of course -- that is easily spotted in our lit Alaskan night skies. When viewing Saturn with binoculars look for its rings and its largest and brightest moon Titan.

Jupiter appears in the night sky too but it’s in the southern portion of the constellation of Ophiuchus, which is near Scorpius and since Scorpius is such a low Zodiac constellation Jupiter also appears very low on the southern horizon around 2 a.m.

The other planets are currently too close to the sun and it’s very hard to view them, especially since they only appear during our long dawn hours.

In the east find Arcturus by following the Big Dipper’s handle, in the Northeast Vega, Deneb and Altair are becoming more prominent. They can be seen all summer during the few hours of semi-darkness and are aptly named the Summer Triangle.

I could talk about the pros and cons of daylight-Saving time but I don’t want to start a discussion that may be futile. Just this

much: local noon is actually at 2 p.m. on the Kenai when the sun is highest, and local midnight at 2 a.m.

Instead I figured I’d talk about religious feasts that are based on astronomical observations of our moon’s phases -- not that that wouldn’t start a discussion.

The Islamic calendar is completely based on the lunar cycle with 12 months, i.e. about 354 days long, each starting with the first sighting of the lunar crescent after the new moon, from Muharram to Dhu al-Hijjah. That’s on average 11 days shorter than the solar-based Gregorian calendar, so on it Islamic holy days are seemingly shifting by 11 days every year, although within the Hijri calendar itself they are celebrated on fixed days. The first year started in 622 (Gregorian calendar) with the Hijra (withdrawal of Muhammad from Medina), so we now have 1428 AH with Ramadan -- the ninth month -- starting on Sept. 12 (the new moon will be Sept. 11) and ending on Oct. 11.

The Jewish calendar is based on the lunar cycle with 12 months basically alternating between 29 and 30 days (our moon needs 29.53 days to go through its cycles), each starting with the new moon, from Nisan to Adar. That means each lunar year is about 11 days shorter than a solar year, so about every third year contains a leap month, Adar II with 29 days. Because that’s still a little too short, leap years are still more frequent (calculated by year modulus 19 as years 0, 3, 6, 8, 11, 14 and 17), hence the “about.” The equivalent in the Gregorian calendar to the Jewish calendar’s first month Nisan is either March or April. In Gregorian calendar 2007 (Jewish calendar 5767), 1 Nisan is March 20 (the new moon was March 19) and with the Passover always beginning on 15 Nisan, that would be April 3. Of course Jewish days start at sundown, so Pesach (Passover) lasts from April 2 through April 9 or 10.

The Christian calendar, more commonly called the Gregorian calendar, is based on the solar year. The date of Easter is computed by figuring out the beginning of spring, i.e. the vernal equinox, figuring out the following full moon and then the upcoming Sunday. That’s the basic computation to compute this date on a computer, hence the official name of Computus, which derives from the Council of Nicaea. This year’s spring equinox happened March 21, the next full moon is April 2 and the next Sunday is April 8.

By the way, the full moon April 2 is the smallest of the year since our moon happens to be near its apogee at 255,000 miles, the farthest point from Earth in its orbit, compared to an average 240,000 miles.

Andy Veh is the physics and astronomy instructor at Kenai Peninsula College. He can be reached at aveh@uaa.alaska.edu.



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