Venus And Mars Charm At Dusk

Venus (center) and Mars (top left) catch the eye at dusk last night Dec. 17 about an hour and 15 minutes after sunset. Venus is smack in the middle of Capricornus, with Mars now in Aquarius. The two planets are about a fist and a half apart. Credit: Bob King

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.

Venus and Mars remain near each other in the dusk sky now through the end of January. Stellarium

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.

The current phases and approximate size comparison of Venus and Mars. Both are gibbous, but Venus is waning to a “half-moon” while Mars is waxing to full. Stellarium

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.

This diagram depicts key spots in an outer planet’s orbit: opposition, when it’s opposite the sun; conjunction when it lines up with the sun and so on. Mars is currently near eastern quadrature. From our perspective, we see around its dark backside a little bit, the reason it shows a gibbous phase at the moment. Click for an animation. Credit: ESO

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.

16 Responses

  1. Jesus Salcedo

    I remember leaving you a comment back in 2012 with fears of the end of the world, I’m 21 years old now. just wanted to thank you for informing me and I want to just give you praise for always informing us and giving us straight facts that are credible!

    1. astrobob

      Thank you, Jesus — I’m glad to help. Thank you for writing, and I hope you’re finding inspiration and enjoyment in the night sky.

        1. astrobob

          Thank you, BC. Merry Christmas from the other side of the border. Hope you get clear skies for next year’s total solar eclipse!

  2. kevan Hubbard

    Venus as a UFO interesting but it doesn’t move however the observer does creating the illusion of movement, holes in the clouds are good for these pseudo movements.Jupiter, morning skies now,can probably be mistaken for a UFO too.

    1. astrobob

      Good points! Holes in the clouds are excellent at creating that illusion. Someone once wrote me about another “UFO” — it turned out to be Sirius — that he saw move because he watched it for many minutes. The person was unaware that stars move over time due to Earth’s rotation.

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