A reader asks, “Every month, when you say the sun is somewhere in the zodiac (like my sign, Pisces), your dates are about a month different from astrology dates. What gives? Is this Ned Green’s fault? Baffled in Boulder.”

Dear Baffled: Yes indeed, the astronomically correct zodiac dates differ from astrological dates by about a month. And nope, Ned Green didn’t do it. Here’s the scoop.

The sun moves through 12 zodiacal constellations (plus Ophiuchus, the Snake Handler, nestled with Scorpius) every year. About 2,100 years ago, in 100 B.C., when astrology was considered legitimate science, Greek astronomers worked out dates when the sun was in each zodiacal constellation.

Constellation sizes vary, but Greek astronomers evened out the zodiacal months by arbitrarily assigning 30 or 31 days to each and calling them “signs.”

Right off the bat, then, astrological “sign” dates did not quite match astronomical reality. Still, the 100 B.C. zodiacal sign dates were nearly accurate.

But then Nature did something tricky.

The earth, like me, has an equatorial bulge (due to rotation, not lack of exercise). Gravity of the moon and the sun, tugging at that bulge, gradually shifts the direction of the earth’s spin axis. This torque doesn’t change the earth’s 23.5 degree tilt from a right angle with its orbital plane. But the -direction- the axis points in space does gradually rotate, in a big circle! This is called precession, like when a toy gyroscope’s spin axis traces out a slow, conical motion. (BTW, the Greek astronomer Hipparchus identified earth’s precession around 150 B.C.)

The earth’s precession cycle lasts 26,000 years, changing our North Star over time. In 100 B.C., there was -no- visible star at True North. Polaris is currently our North Star, but eventually our spin axis will drift away from it. In 12,000 years, Vega will be the North Star. And in 26,000 years Polaris will return as the North Star.

Precession also shifts the sun’s position through the zodiac, at about one day every 72 years, or one month every 2,160 years. The sun’s zodiacal positions have thus changed by a month since 100 B.C.

Astrological “signs,” which didn’t quite match the sun’s positions even in 100 B.C. but were at least in the ballpark, now differ from the sun’s true astronomical positions by a month.

Meaning that, if your birthday is March 1, your arbitrary astrological “sign” is still Pisces per the situation of 100 B.C. But your birthday’s true solar constellation position is now Aquarius. The diagram (which introduces our column’s new resident astronomer, Luna, and her dog Sirius) shows currently accurate solar zodiacal dates (lowest line) relative to astrological “sign” dates (uppermost line) that are now 2,100 years out of sync with reality.

Backyard Astronomy Lecture Series: This columnist is conducting a lecture series, Backyard Astronomy, at the Nederland Public Library on the first Wednesday evenings of May through August, with a bonus talk, How Far is That Star, on the first Wednesday of September. Details and registration at https://nederland.colibraries.org/ backyard-astronomy/. (Like the zodiac, the series was shifted back a month due to the columnist’s having had a cold in April.)

Also in May skies: The sun begins the month in Aries, entering Taurus on May 13. At mid-month, days and nights are 14 and 10 hours long, respectively.

The moon’s dates are: Full (Flower Moon) May 5; Last Quarter May 12; New May 19; First Quarter May 27.

May Meteor Showers: Eta Aquariids (parent, Halley’s Comet ) and Eta Lyrids (parent, comet C/1983 H1, IRAS-Araki-Alcock) peak on May 6 and 9 respectively. Aquariids are visible in Nederland between 3 a.m. to sunrise; Lyrids after 11 p.m.

Best Sky Viewing Nights (Minimal Moon): May 12-27.

Sunset (Mid-Month): Venus is spectacularly bright, high in the west at sunset. Leo with Regulus is prominent in the south. Ursa Major (the Big Dipper) is nearly overhead.

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

Sunrise (Mid-Month): The Great Square of Pegasus is in the east. Arcturus sets just as the sun rises.

Mercury is mostly lost in the sun. It might be seen, with difficulty, very low in the east just before sunrise on May 29.

Venus is the Evening Star, at maximum elevation above the horizon on May 12. It will make a striking show with a thin sunset crescent moon on May 23.

Mars is just leftward of Pollux and Castor, the Gemini Twins, in the evening sky. It sets by about midnight.

Jupiter is hard to spot, low in the east just before sunrise.

Saturn is low in the eastern sky, but now easily visible, before dawn.

Notable Space Missions: An innovative NASA constellation of small-sats called TROPICS, which will look deep inside violent storms, launches on May 1. China plans to launch a newly designed, automated cargo ship called Tianzhou to its crewed space station on May 10. A National Reconnaissance Office signals intelligence satellite, Orion 11, is slated for launch into geosynchronous orbit.

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