SCAS – Southern Colorado Skies / August 21 – 27, 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.

MARS RETROGADE

Normally Mars appears to move from west to east against the night sky, but for a couple months every two years or so Mars seems to move from east to west. Ancient astronomers were befuddled by this action, which caused Ptolemy (famous for his ‘earth-centered’ model of the solar system) to add “epicycles” (circles within circles) to his model to explain this aberrant behavior – called “retrograde” motion. (Retrograde also occurs with Jupiter and the other outer planets – Venus and Mercury also exhibit retrograde motion during inferior conjunction, i.e., while in the Sun’s glare so we can’t observe it!)

With the acceptance of the Copernican ‘Sun-centered’ model, we can now see that it’s all an illusion. Referring to the attached graphic, imagine Mars and Earth on an oval race track, where Earth occupies the inside lane and moves faster than Mars (from Kepler’s third law we know that the closer to the Sun, the faster the planet moves). In fact, Earth completes about two laps for every one lap by Mars, and every 26 months or so Earth comes up from behind and passes Mars. Mars and Earth don’t quite occupy the same orbital planes, so if you were to “connect the dots” every night during retrograde you would see an open zigzag pattern, as in the graphic, or a closed loop, depending on where Mars and Earth happen to be in their tilted orbits.

retrograde2

 

 

 

 

 

 

 

 

THIS WEEK

Monday, August 22

Venus and Jupiter care only about 6 degrees apart and moving closer to conjunction this Saturday (see below). Look just above the horizon due west at dusk. You might try binoculars for a look at Mercury, which is about 4 degrees below Jupiter.

Tuesday, August 23

Saturn, Mars, and Antares make a nearly straight, nearly vertical line in the south-southwest just after dark.

Wednesday, August 24

Saturn, Mars, and Antares again make a nearly straight, nearly vertical line in the south-southwest just after dark.

Last-quarter Moon at 9:41 p.m. MDT.

Thursday, August 25

Today you may be able to see the Moon occult Aldebaran, through a telescope and with haze-free skies. Aldebaran will disappear about 11:43 a.m. then reappear about 12:14 p.m. (Denver times).

Friday, August 26

Venus and Jupiter are now only 1 degree apart, a ‘teaser’ for the big show tomorrow.

Saturday, August 27

Venus-Jupiter conjunction! About 20 minutes after sunset, look low above the horizon due west (left of where the Sun went down). The two planets will be about 0.1 degree apart. That’s so close you may need binoculars to see that there are two objects, not one!

PLANETS

Mercury, Venus, and Jupiter

Look for this trio low in the twilight about 20 or 25 minutes after sunset. Jupiter and Venus (the brighter of the two) will be very low on the horizon, west (left) of where the Sun just disappeared. They were about 8 degrees apart on August 19th and are approaching each other about 1 degree per day. Binoculars will help if you have a pair. The grand finale is on August 27th, when Jupiter and Venus will be less than 0.1 degree apart!

For much fainter Mercury, look to about 4 or 5 degrees below Jupiter.

Mars and Saturn

Along with Antares, below Saturn, the three form an almost straight, vertical line on August 23rd and 24th.

Uranus (in Pisces)

High in the east after midnight.

Neptune (in Aquarius)

High in the southeast after midnight.

BRIGHT IRIDIUM FLARES

Wednesday (August 24): 09:37 p.m. / Magnitude -4.3 / E / elevation 45 degrees

Thursday (August 25): 05:59 a.m. / Magnitude -6.8 / NNE / elevation 31 degrees

Thursday (August 25): 09:33 p.m. / Magnitude -5.1 / E / elevation 46 degrees

Saturday (August 27): 05:47 a.m. / Magnitude -3.2 / NNE / elevation 28 degrees

INTERNATIONAL SPACE STATION (ISS)

Sorry – these apparitions 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

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