Archive

Posts Tagged ‘Precession’

Ephemeris: 03/13/2026 – Precession of the equinoxes

March 13, 2026 Comments off

This is Ephemeris for Friday, March 13th. Today the Sun will be up for 11 hours and 47 minutes, setting at 7:46, and it will rise tomorrow at 7:56. The Moon, 2 days past last quarter, will rise at 6:01 tomorrow morning.

Yesterday I talked about the fact that the constellation of Cancer the crab is no longer the northernmost constellation of the zodiac, where the Sun is positioned on the first day of summer. There’s been a slow change in the position of the earth’s axis, in that it wobbles slowly in a period of nearly 26,000 years. The inclination of the Earth’s axis to its orbit is around 23 1/2 degrees, and the gravitational pull of the Moon and the Sun on the Earth’s equatorial bulge, wants to straighten it up. But because the Earth is spinning, it wobbles instead, slowly sliding the actual zodiacal constellations eastward about one constellation in 2,150 years. So the actual constellations are off one constellation from the astrological signs.

The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EST, UT – 5 hours) unless stated otherwise. Times will be different for other locations.

Addendum

An animation of the precession of the equinoxes.
An animation of the precession of the equinoxes. The blue horizontal line is the celestial equator. The orange line is the ecliptic, the apparent annual path of the Sun against the stars. Where the two lines cross is the vernal equinox where the Sun is on the first day of spring, which on our calendar is trending to be March 20th. The slippage of the stars eastward (to the left) along the ecliptic is about the apparent width of the Sun or Moon, or half a degree, in 36 years. We’re looking at two different years 150 CE, the time of Ptolemy, and our time. To tell which is which, the one from our time has Saturn at the lower right. Jupiter happens to be in both of them, but it’s obviously been around lots of times between then and now. Created using Stellarium and GIMP.
The force causing precession
The Moon and Sun’s gravitational force act on the Earth’s equatorial bulge attempting to cause the earth to straighten up and fly right. Because the Earth is spinning, it acts like a gyroscope and the torque to straighten it up causes it to be applied 90 degrees away in the direction of the rotation causing the procession. Credit: Open Course: Astronomy.
Precession circle
The path of the north pole of the sky (celestial sphere) over time. The celestial north pole is still approaching Polaris. In the past the Big Dipper was closer to the north pole of the sky than it is now. Polaris will be it closest to the north pole around the year 2110. Source: taichifuture.com/cosmology.html.
Categories: Concepts, Zodiac Tags:

06/19/2023 – Ephemeris – Juneteenth and the flight to freedom

June 19, 2023 Comments off

This is Bob Moler with Ephemeris for Juneteenth, Monday, June 19th. Today the Sun will be up for 15 hours and 34 minutes, setting at 9:31, and it will rise tomorrow at 5:56. The Moon, 1 day past new, will set at 11:22 this evening.

In 1865, it was on June 19th that the Union soldiers, led by Major General Gordon Granger, landed at Galveston, Texas with news that the war had ended and that the slaves were now free. Until then about the only path to freedom for the slaves was “To follow the Drinking Gourd”, or the Big Dipper, northward. The Big Dipper and the Great Bear, Ursa Major, has, for the last few millennia been the constellation of the north, being closer to the North Pole of the sky than it is now. The Earth’s axis has a slow wobble, called precession, caused by the gravitational pull of the Moon and Sun on Earth’s slight equatorial bulge of 30 miles wider than the pole to pole distance. That wobble period is nearly 26 thousand years.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT –4 hours). They may be different for your location.

Addendum

The path of the north pole of the sky (celestial sphere) over time. The celestial north pole is still approaching Polaris. In the past the Big Dipper was closer to the north pole of the sky than it is now. Polaris will be it closest to the north pole around the year 2110. Source: taichifuture.com/cosmology.html.
Runaway slaves following the Drinking Gourd
“Follow the Drinking Gourd,” a show at the Virginia Living Museum’s Abbit Planetarium based on Jeanette Winter’s children’s book of the same name, tells the story of a slave family who used the stars to make their way north. Visitors can watch the free show Saturdays in February. (Courtesy of Virginia Living Museum) Via Daily Press, Newport News, VA.
Categories: Constellations, History, Holiday Tags: , , , ,