Diagram of Easter dates for 2021-2061 for Western churches. Easter’s dates exemplify a solar-lunar algorithm. Their sequence never repeats, but it does form patterns. Diagram by Frank Sanders for The Mountain-Ear.[/caption]

Easter falls on April 9 this year. It is called “moveable” because its date shifts yearly. This month, we look at moveable calendar dates.

There are three basic calendar types: solar, lunar, and hybrid solar-lunar. Solar calendars are locked to the earth’s annual trip around the sun. The best-known of these is the Gregorian, initiated in 1582 (but not adopted in Britain and its American colonies until 1752) and now used worldwide for civil purposes. It has 365 days with an extra leap day (February 29) in each year divisible by four. But for closer sun synchronization, three leap days are strategically -omitted- out of every four centuries, and leap days in the years 4000, 8000, etc., are also omitted. This holds the Gregorian’s (Common Era, CE) March 21 to within a day of the astronomical northern spring equinox for 20,000 years.

The Islamic calendar exemplifies the purely lunar model, in which 12 successive lunations (first crescent to first crescent) are a year. This makes the Islamic year 354 or 355 days, shorter than a solar year by about 11 days. The Islamic year count begins in 622 CE at Anno Hijra 1, AH 1. Its year’s shortness. Its year’s shortness relative to solar years means that it gradually gains on the CE year count. We’re now at AH 1444; near 20,900 the AH and CE year counts will pass through each other.

The Jewish, Chinese, and Hindu calendars are solar-lunar. These systems use a solar anchor point such as a solstice or an equinox, and then a notable calendar date occurs on some subsequent lunation. For instance, the Chinese New Year begins at the second new moon after the winter solstice (landing it between January 20 and February 21, Gregorian).

Easter is evolutionarily related to the Jewish solar-lunar Passover date of Nissan 14. It has been defined, from the synodical conferences of Nicaea and Whitby in 325 and 664 CE, to be the first Sunday that falls after the first full moon that itself falls either on or after the northern spring equinox, taken as March 21. The accompanying graph shows 40 years of Western-church Easter dates. Easter’s dates are non-periodic, but this chart demonstrates that they do form patterns.

It can get more complicated. The Chinese calendar correlates Jupiter’s movement with 12 zodiacal signs. The Mayan calendar might win the prize for most-sophisticated, though, by interlocking solar and lunar periods with Venusian 584-night cycles. Outstanding.

The commonality of all calendars is that they are entirely human, arbitrary constructions. Pick your favorite or try making up one of your own; they give us variety and they are all good.

Backyard Astronomy Lecture Series: This columnist will conduct a four-part lecture series, called Backyard Astronomy, at the Nederland Public Library on the first Wednesday evenings of April through July, with a bonus talk, How Far is That Star, on the first Wednesday of August. Find details and get registered at https:// nederland.colibraries.org/backyardastronomy/.

Also in April skies: The sun begins the month in Pisces, entering Aries on April 19. At mid-month, days and nights are 13 and 11 hours long, respectively. On April 15, solar time equals clock time; in Nederland the sun will be due south at 12:02 p.m. Standard Time (1:02 p.m. Daylight Saving Time).

The moon’s dates are: Full (Pink Moon; Paschal (PAS-k’l) Moon) April 6; Last Quarter April 13; New April 20; First Quarter April 27.

April Meteor Showers: Lyrids (parent, Comet Thatcher) and Pi Puppids (parent, Comet Grigg- Skjellerup) peak together on April 23-24. They are visible in Nederland between 10 p.m. to sunrise.

Best Sky Viewing Nights (Minimal Moon): April 13-27.

Sunset (Mid-Month): Venus is the Evening Star, in the west at sunset. Leo with red Regulus is in the southeast, with the Big Dipper in the northeast. Procyon (Rises Before the Dog) is due south. Sirius (The Dog) and Orion are in the southwest.

Midnight (Mid-Month): Vega, Deneb and Altair form the Navigator’s Triangle low in the east to northeast. Follow the Big Dipper’s arm, arcing to Arcturus and then on to Spica. Gemini is low in the west.

Sunrise (Mid-Month): The Navigator’s Triangle is overhead. Scorpius with bright red Antares is prominent in the southwest. Spica, in Virgo, sets in the southwest.

Mercury is at maximum elongation from the sun with its best visibility of the year, low in the west, just after sunset on April 11. (The west end of Barker Reservoir affords a good place to see it.) See if you can spot it next to the fresh crescent moon on April 21.

Venus is the Evening Star, in the west at sunset. It will form a nice conjunction with the crescent moon on April 23.

Mars, in the southwest below Castor and Pollux at sunset, sets around 1:45 a.m.

Jupiter has disappeared in the sun.

Saturn rises just before the sun; try to spot it, low in the east, in the pre-sunrise hour.

Notable Space Missions: CU’s Laboratory for Astronomy and Space Physics (LASP) Colorado Inner Radiation Belt Experiment (CIRBE) space-weather probe launches from Vandenburg AFB on April 9. The European Space Agency’s Jupiter Icy Moons Explorer (JUICE) mission is set for launch from Kourou, French Guiana, on April 13. JUICE will take 11 years to reach, and enter orbit around, the Jovian moon Ganymede. Boeing’s Starliner capsule should make its first crewed flight. Astronauts Barry Wilmore and Sunita Williams will shake it out during seven days in orbit. SpaceX is still “a month away” from the first orbital flight, from Texas, of its 400-foot reusable Starship.

Frank Sanders, a spectrum scientist at the U.S. Department of Commerce in Boulder, takes astronomy-related inquiries at backyardastronomy1@gmail.com.