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The Sky This Week, 2011 April 19 - 26

The Easter "computus", springtime stars

Saturn, imaged 2011 APR 15, 02:53 UT
South at top.  Note bright spots in northern hemisphere cloud belt

The Moon wanes in the late night and early morning skies this week, with Last Quarter occurring on the 24th at 10:47 pm Eastern Daylight Time. Luna spends most of the week skulking around the southern reaches of the Ecliptic among the stars we’ll be seeing later this summer. Look for the ruddy star Antares in the constellation of Scorpius just over five degrees to the right of Luna’s waning gibbous on the morning of the 21st. From there she will wend her way through the "teapot" asterism of Sagittarius before entering the star-poor region of the rising autumnal constellations.

Two of the world’s major religions celebrate major observances this week, both of which are tied to the first full lunation of the spring. Both Jewish Passover and Christian Easter are set by the Full Moon that follows the vernal equinox, more of less in keeping with traditions that date back at least two millennia. In the case of Easter a formula known as the "computus" has been used since medieval times to set the date of Christendom’s most important feast, and over the years a few astronomical rules have been "bent" to make the task somewhat easier. "Conventional wisdom" tells us that Easter occurs on the first Sunday following the first Full Moon that follows the vernal equinox. In practice this is true most of the time, but the "computus" makes certain non-astronomical assumptions which sometimes can alter this outcome. First of all, it assigns the equinox to March 21, even though astronomically it occurs most frequently on the 20th these days. Next, the "computus" determines the Full Moon by a tabular count of 14 days into a lunation even though the phase may fall on the 15th calendar day after the lunation began. Easter cannot occur any earlier than March 22 or any later than April 25, and it’s usually one of these extreme dates that comes about because of the ecclesiastical rules. This year’s late Easter is actually pretty conventional, though, but it just happens to occur very late in the possible date range. It hasn’t fallen this late in the season since 1943, and it won’t occur this late again until 2038!

The end of evening twilight finds the winter constellations in the process of setting while two of spring’s signature constellations grace the meridian. To the north you should have no trouble spotting the seven-star asterism of the Big Dipper as it wheels around the north celestial pole. This is the best time of year to find this distinctive group, one of the best-known in the northern sky even though it doesn’t contain a single first-magnitude star. To the south you’ll see a lone bright star on the meridian surmounted by a loop of fainter stars that give the whole the appearance of a sickle. The bright star is Regulus, heart of Leo, the Lion, and the rest of the "sickle" forms the lion’s head. Trailing slightly to the east is a right triangle of second-and third-magnitude stars that form the lion’s hindquarters. Leo is one of the oldest Zodiacal constellations and one of the few that actually resembles its namesake.

Saturn is now well up in the southeast by 10:00 pm and presents a wonderful sight in small to medium-sized telescopes. The planet’s trademark rings are tilted about 8 degrees to our sight, and a good six-inch telescope should easily show the dark line of Cassini’s Division, a gap in the ring system caused by gravitational resonances with Saturn’s inner moons. To give you a sense of scale, the division spans a distance roughly equal to the Earth’s diameter!

Early risers can still catch dazzling Venus rising shortly before the Sun. She can only be seen in the gathering brightness of morning twilight now, but she remains quite visible up until Old Sol actually crests the horizon. On particularly clear mornings you might be able to keep her in view for an hour after sunrise as well. That’s a harbinger of a nice spring day!

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