SCAS – Southern Colorado Skies / Sepember 4 – 10, 2016

Consider a planetarium program to help you plan your observations. There are many commercial products available, and a very good program (Cartes du Ciel) is available free at http://www.ap-i.net/skychart/en/start.

PRECESSION

The term ‘precession’ refers to a very slow shift in the Earth’s axis of rotation – this motion has often been compared to a wobbling top (see figure). In the case of Earth, this wobble forms a complete revolution in about 26,000 years (25,772 years, to be more precise). This phenomenon has been called (in a non-technical context) the “precession of the equinoxes,” because it causes the equinoxes to slowly drift through the zodiac along the ecliptic. Amazingly (to me, anyways), the discovery of the precession of the equinoxes is generally attributed to the Greek astronomer Hipparchus – in 129 B.C.! There are even claims that the discovery was first published in an Indian text about 700 B.C. !

precession1a

Precession is caused by the gravity of the Sun and the Moon acting on Earth’s rotation (the other planets exert a small influence as well). Projecting onto the Earth’s sky at the north pole (see figure) shows the effect the wobble has on the position of the north star; the north celestial pole traces out a circle in the sky over 26,000 years. Today the north celestial pole points to within about 1 degree of Polaris; in about 12,000 years the north celestial pole will point to about 5 degrees from Vega. It will again point closest to Polaris in about 26,000 years.

precession2_df

THIS WEEK

Wednesday (September 7)

The asteroids 1 Ceres and 18 Melpomene, magnitudes 8 and 9, respectively, are just 0.8 degrees apart. They will be visible in a telescope or binoculars very late tonight and tomorrow night. See http://www.skyandtelescope.com/observing/celestial-objects-to-watch/ceres-and-melpomene-in-binoculars/.

Thursday (September 8)

Look for the Moon tonight as it joins Saturn and Antares in a nearly vertical lineup. That’s Saturn and Antares below the Moon, and brighter Mars to their left (east).

PLANETS

Mercury (in Virgo)

Hidden by the glare of the Sun.

Venus (in Virgo)

Very low in the west about 20 or 30 minutes after sunset.

Mars and Saturn (both in Ophiuchus)

Mars continues to move to the left (eastward) and away from Saturn and Antares (below Saturn). Look in the south-southwest at nightfall.

Jupiter (in Virgo)

Currently out of sight in the Sun’s glare.

Uranus (in Pisces)

High in the southeast after midnight.

Neptune (in Aquarius)

High in the south after midnight.

BRIGHT IRIDIUM FLARES

Alas, there are no particularly bright Iridium flares predicted for this week.

INTERNATIONAL SPACE STATION (ISS)

These are too numerous to list here! If you’re serious, load the ISS DETECTOR app on your smart phone or tablet.

Have fun!

Dave Furry, SCAS Director of Education

SCAS_Web_logo

Leave a Reply