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Solstice – the day the sun stands still

Greetings, stargazers.

I hope you were in a good location to see the lunar eclipse last month. I saw only clouds, but a friend of mine got to watch the eclipse from the South Pole. That brought up a brief discussion of moonrise and moonset as viewed from the pole. In most parts of the world, the moon rises and sets every day, just like the sun.

But at the South Pole, while it is circling the sky near the horizon, moonrise takes a week, then it takes a week to set, and then there are two weeks of darkness. This is because the moon’s orbit is tilted with respect to the Earth’s rotational axis. It takes about a month to complete an orbit, with half the time above the horizon, and half the time below it.

Similarly, as viewed from the pole, the sun takes three months to rise, three months to set, and then there are six months of darkness. As it is rising and then setting, the sun doesn’t abruptly change directions, but slowly turns around to head the other way. At the peak of motion, the sun’s path appears to stop briefly, or “stand still” before it turns around. Combining the Latin “sol” for sun and “sistere” for stand still, we get solstice. The summer solstice this year is on June 20. While Durango has extra-long days, the South Pole is in the middle of its six-month night.

Earth’s axial tilt causes our seasons, and the 27 degree tilt determines how far the sun goes north and south during the year. The midpoint is at the equinox when the sun is directly over the equator. If you look to the south (from the Northern Hemisphere), the sun rising in the east and setting in the west will make a large clockwise arc through the sky. This is where the term clockwise comes from, as the location of the sun in the sky is the original clock.

This month

The Big Dipper is close to its highest point in the sky right after dusk. The bucket of the dipper will be toward the north, with the two end stars, Duhbe and Merak pointing toward Polaris. The handle is pointing up toward zenith. If you follow the arc of the handle, it will “arc to Arcturus, and speed on to Spica.”

Arcturus is one of the brightest stars in the sky and will be about 30 degrees from the end of the handle and a bit to the south of zenith. Spica is another 30 degrees along the arc. It is not as bright, but it’s the brightest object around in the middle of the southern sky.

The summer triangle is in the northeastern sky right after sunset. This triangle is a bit larger than the Big Dipper and is made up of Vega, Deneb and Altair. Vega is the brightest star in the triangle. It is just as bright as Arcturus, but noticeably bluer in color because it is much hotter than Arcturus. Deneb will be below Vega to the northeast, and Altair, the third star, will be rising almost due east.

If you have a clear view to the south, this is the time of year you can see Omega Centauri from the Four Corners. It is the most massive globular cluster in the Milky Way and contains an estimated 10 million stars. It will only be 5 degrees above the horizon and faintly visible to the naked eye as a fuzzy spot. With binoculars or a small telescope, it is one of the most interesting objects in the sky to see.

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

OMEGA CENTAURI: https://en.wikipedia.org/wiki/Omega_Centauri.

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

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


FOUR CORNERS STARGAZERS: https://bit.ly/2pKeKKa.