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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Easter by the Computus
by Geoff Chester, USNO Public Affairs
12 April 2022
The waxing gibbous Moon, imaged 2022 April 11 from Alexandria, Virginia
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor, iOptron AZ Mount Pro,
1.6X Antares Barlow lens, and a ZWO Optical ASI183MC CMOS imager.
The Moon brightens the evening sky this week as she wends her way through the springtime Zodiacal constellations. Full Moon occurs on the 16th at 2:55 pm Eastern Daylight Time. April’s Full Moon is popularly known as the Pink Moon, a name probably derived from the appearance of pink phlox wildflowers that bloom across much of North America at this time of the year. Other names are also indicative of the arrival of spring, so it is also known as the Seed Moon, Budding Moon, and the Egg Moon. The latter name, derived from Anglo-Saxon folklore, may be the origin of Easter eggs as symbols of the season. Luna may be found in the vicinity of the bright star Regulus in Leo on the evenings of the 12th and 13th. On the 15th and 16th she lies near the bright star Spica in Virgo.
The Full Moon on the 16th is also the Paschal Moon, which means that the following Sunday, the 17th, marks the date of Easter, the most important “moveable feast” in the Christian calendar. Determining the date of Easter is thus of paramount importance to the faithful, and the formula used to make this calculation relies on ecclesiastical rules that are over 1500 years old. Most people assume that Easter falls on the first Sunday following the first Full Moon following the vernal equinox. In general this is a workable “rule of thumb”, but 1500 years ago simple math revolving around well-known cyclical events allowed local priests to calculate the date of Easter with the same precision as papal astronomers in Rome. The formula they used, known as the “Computus”, made certain assumptions that were not necessarily based on the reality of the sky. First of all, at the Council of Nicaea in 325 CE, the equinox was decreed to be March 21st. Over the ensuing centuries a number of calendar cycles were incorporated into the Computus to set the date of the Paschal Moon. If you look in an astronomical almanac you will find terms like the Dominical Letter, Golden Number, Roman Indiction, and Epact. A combination of these cycles leads to the calculation of the date of the Paschal Moon, which may not occur on the same date as the astronomical Full Moon. This is why, in some years, Easter falls on the “wrong” date if you strictly follow the astronomical timing of the spring’s first Full Moon.
As Luna waxes toward her full phase her light washes out many of the stars of the spring constellations. Unlike winter’s constellations, which contain nine of the sky’s 25 brightest stars, the spring sky shows just a handful of luminaries scattered over a large swath of sky. One star that stands out prominently among the rising constellations is Arcturus, the lead star in the constellation of Boötes, the Herdsman. You will find Arcturus, characterized by a pale rose tint, rising in the east at the end of evening twilight. As the brightest star in the northern hemisphere of the sky and the fifth-brightest overall Arcturus is very hard to miss. It is relatively nearby at a distance of 36.7 light years. It’s reddish hue indicates that it is an evolved star that has exhausted the hydrogen in its core and is becoming a bloated “red giant”. It is the closest star of this type to the solar system and gives us an idea of the fate of our Sun some 2.5 billion years from now.
The bright planets are still best seen in the pre-dawn sky. A few weeks ago Venus, Mars, and Saturn were clumped together in a series of interesting conjunctions, but now they are more like beads on a string. The first to rise is yellow-tinted Saturn, followed by the ruddy glimmer of Mars. Next up is the brilliant dazzle of Venus, followed by the bright glow of Jupiter, which should be visible very low in the southeastern sky at 6:00 am local time. Venus and Mars will advance eastward toward Jupiter as Jupiter rises earlier each morning. The two bright planets will pass close to each other by the end of the month.
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