Clear and cold nights are perfect for early evening observing. Get a quick look then duck back in to warm up. If you’ve been wondering what that big, bright “star” is in the southwest after sunset, it’s Venus, brightest of all the planets. If you know where to look, you can even spot the stunning gem before sunset. We’ll talk about how to do it in a moment, but first let’s admire the brilliance of this planet. Easily visible 30 minutes after sundown, Venus now resides in the zodiac constellation, Capricornus the sea goat and remains visible till around 730 p.m. local time, well past the end of evening twilight.
Not quite two fists to the upper left of Venus, Mars shines meekly in comparison but possesses a distinctly reddish hue. Can you guess how much brighter Venus is than Mars? Five times? Ten times? They’re currently five magnitudes apart in brightness. Since each magnitude step is 2.5 times brighter than the next, that comes to 2.5 x 2.5 x 2.5 x 2.5 x 2.5 = 97.6. Call it 100. So Venus is 100 times brighter than Mars!
One reason why is that Venus is 100% cloudy, so it reflects much of the light it receives from the sun. Another reason is distance: Venus is only 80 million miles (129 million km) away, Mars is 144 million (232 million km). Yet another reason is size: Venus is 7,521 miles (12,103 km) in diameter and Mars 4,212 miles (6,778 km). Mars also has a smaller surface area from which to reflect light.
I put the scope on Venus and enjoyed seeing its phase, now 62% illuminated, oval-shaped and looking like a waning gibbous moon. Mars was much smaller, so I had to use higher magnification to see anything at all. Earth and Mars were closest in May (47 million miles away), when the Red Planet appeared larger and showed lots of detail to the patient eye. The two planets have since parted and Mars has shrunk! Although no details were seen on its glaring pink disk, its phase was obvious in the scope, a waxing 3/4 or gibbous.
Because Mars orbits the sun outside of Earth’s orbit, it typically appears like a tiny full moon in contrast to the inner planets Venus and Mercury, which can show all phases from crescent to full. They’re able to do this because they can appear on the opposite side of the sun as well as between the Earth and sun. Mars, with an orbit outside of Earth’s, can never come between our planet and the sun.
But when the planet lies at a right angle to Earth and the Sun, called quadrature, we see around into his shadowed side a bit, and Mars appears slightly shaded instead of completely full and round. To see Mars’ phase, you’ll need a telescope that can magnify about 100x. For Venus, 30x will do because the planet appears much larger. To find Venus before sunset, line up the planet with a landmark after sunset and then look slightly above and left of that spot shortly before sunset.
It’s nice to have a big planetary light at Christmas, just one of the reasons seeing Venus makes me happy inside these nights. Watch as the planet and Mars gradually approach one another over the next 6 weeks. They’ll reach a minimum separation of just 5° in late January before Venus makes a hard right and heads back toward the sun.