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The difference between a tropical year and a sidereal year

Greetings, stargazers.

The autumnal equinox this year is at 7:03 p.m. Sept. 22. We typically think of the first day of fall as something that happens all day long, but the equinox is the time when the sun crosses the celestial equator, and that always happens at a specific point in time. The length of time between equinoxes is called a tropical year.

Next year, the equinox will be on Sept. 23. This occasionally changing of equinox days is simply because of leap years and the fact that a tropical year is almost a quarter of a day longer than 365 days. We have leap years so our seasons will stay consistent over the decades, centuries and millennia.

What is interesting about a tropical year is that it is not the same as the time it takes for the Earth to make a full orbit of 360 degrees around the sun. A sidereal year is the time it takes for the sun to return to its same position compared to the background stars, and this takes a little over 20 minutes longer than a tropical year. In a practical sense, the stars are in a slightly different place from one year to the next, and the effect is called the precession of the equinoxes. But it is a very small effect – it takes about 73 years to shift 1 degree.

The difference between a tropical and a sidereal year was first measured by Hipparchus in the second century B.C., which was quite an accomplishment for such a subtle difference. It would take centuries for most of us to notice. An example today is that the sun enters one of the constellations of the zodiac much later than the dates specified for a given horoscope sign. Although you are considered to be a Virgo if you were born between Aug. 23 and Sept. 22, the sun doesn’t actually enter the constellation Virgo until Sept. 16, which is almost a month off from when the dates were originally set. In another 73 years it will be off by an additional day.

From a physical perspective, precession can be explained as the axis of the Earth wobbling the same way a spinning top will wobble. The Earth’s wobble takes 25,772 years to complete one cycle. Another, possibly more obvious, long-term effect is that the star Polaris is only the pole star for a brief time. The Egyptians building the pyramids noted the star Thuban in Draco as the pole star. About 13,000 years in the future Vega will be our pole star.

This month

Saturn is joined by Jupiter in the evening sky. Both are in the southeastern sky after sunset, with Jupiter being by far the brightest object in the sky after the sun and moon. Tonight (Saturday), Jupiter and the full moon both rise a little after 8. Jupiter reaches opposition, its closest approach to Earth on the Sept. 26. On that date it will cross the meridian, the imaginary north-south line that passes directly overhead at local midnight. Because of daylight saving time, this will be a little after 1 a.m.

The summer triangle is almost directly overhead in the evening this month. The three bright stars include Vega, Deneb and Altair. Vega, the brightest star in the sky now, will be near zenith at sunset. Deneb will be 24 degrees to the east, and Altair 34 degrees to the south. These bright stars are great reference points for helping you find dimmer constellations using a star chart.

The summer Milky Way is still high in the sky right after sunset, with the brightest part toward Sagittarius near the southern horizon. Scanning the Milky Way with binoculars is one of my favorite stargazing activities.

Last month, I mentioned Comet 2017 C/2017 K2 PanSTARRS, and this month it is near the front of Scorpius. Although it is a bit dimmer this month, it is still heading toward the sun, so its brightness could change any time. You will need a telescope and a daily star chart to locate it.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.

Useful links

HIPPARCHUS: https://en.wikipedia.org/wiki/Hipparchus.

ASTRONOMY PICTURE OF THE DAY: http://apod.nasa.gov/apod.

OLD FORT LEWIS OBSERVATORY: www.fortlewis.edu/observatory.