Around the Sky in 28 Days – Day 2
September 10, 2010
Now things get interesting. For northern observers, the Moon is finally high enough to be seen assuming your view of the southwestern horizon is clear. As the chart below shows, the Moon forms an irregular diamond shape with Venus, Mars, and the bright star Spica.
Venus is the most brilliant “star” of the stellar trio at a spectacular magnitude of -4.7. The 2nd planet from the Sun, Venus is only a little smaller than Earth (0.95 Earth radii, 0.82 Earth masses). Its brightness is due to its highly reflective atmosphere which reflects 65% of the sunlight that strikes it. This atmosphere is a potent one with surface temperatures over 90 times greater than Earth’s. It is also 96% carbon dioxide so Venus is experiencing greenhouse warming “gone wild” resulting in surface temperature over 850°F. Being located on a orbit with a semi-major axis of 0.72 AU, Venus orbits the Sun every 225 days (or 0.62 Earth years).
Much fainter Mars (magnitude +1.5) is the northernmost object in the stellar trio. The 4th planet from the Sun, Mars has with a semi-major axis of 1.52 AU and takes 687 days (or 1.88 Earth years) to orbit the Sun. It is much smaller than the Earth with a radius of 0.53 Earths and a mass of 0.11 Earth masses. Though Mars’ atmosphere is also 95% carbon dioxide, it has lost much of its atmosphere due to its very low gravity (0.38 Earth gravity). As a result, Mars is currently a cold, dry world though recent discoveries have identified large reservoir of water ice below its surface.
Spica is the only non-planet among the stellar trio. The brightest star in the constellation of Virgo and the 15th brightest star in the sky (magnitude +1.0), Spica is a close binary system consisting of a blue giant (10 solar masses, 7 solar diameters, 12,000 solar luminosities) and a blue main sequence star (7 solar masses, 3.6 solar diameters, 1,500 solar luminosities). It is ~260 light years away.
While watching the celestial quartet, try to image the immense distances between all four.
* The Moon is 224,000 miles or 358,000 km away
* Venus is 48.5 million miles or 77.6 million km away
* Mars is 203 million miles or 325 million km away
* Spica is a whopping 1,528,000,000,000,000 miles away or 2,460,000,000,000,000 km away
The light you are seeing took
* 1.2 seconds to travel from the Moon to your eyes
* 4.3 minutes to travel from Venus
* 29 minutes to travel from Mars
* and 260 years to travel from Spica
And for my southern readers, here’s the view from down south. And what a view it is…