Venus Proves That Beauty Can Be Elusive

Venus is the solitary light above the TV and radio towers in Duluth's "antennae farm" last night. Details: 200mm lens at f/2.8, 1/80-second exposure, ISO 800. Photo: Bob King

Heck, it’s the brightest planet, and about as far as it gets from the sun in the evening sky, so how come Venus is hiding in the sunset glow? Checking the stats, I see Venus reached its greatest eastern elongation of 46 degrees – largest distance east of the sun – only a week ago. That’s half the distance between the horizon and the overhead point. Shouldn’t the planet pop right out in the sky?

Nope. Instead, you’ll find Venus very low in the southwestern sky well to the left of the sunset point about a half hour after sunset. If you’re on a lake or standing in an open field, it’s easy to see, but for many of us who live around buildings and trees, it’s easily lost behind a roof line. This is sad, because we can’t fully appreciate how amazingly bright this planet is so long as it can’t step up to the plate.

Venus isn’t to blame. It has no choice but to follow the path all the planets must follow as they orbit the sun. Called the ecliptic, it’s an imaginary circle in the sky that closely defines the flat plane of the solar system. Since all the planets (and moon) orbit in or near this plane, we always see them following the same path through the same 12 zodiac constellations over the days, months and years. Even the sun tracks along the ecliptic as seen from Earth, because we’re driving on the same race track as the rest of the planets. Since we’re stuck on Earth and can’t see ourselves traveling along the ecliptic from afar, the sun appears to follow the ecliptic instead. Watching the sun’s seasonal travels up and down the sky is a reflection of our planet’s orbital motion around the sun. The sun’s not moving, we are.

The changing angle the ecliptic makes to the horizon strongly affects how easily planets are visible when near the sun. The left panel shows Venus tonight when its path follows a long angle to the horizon; the right panel depicts its next period of evening visibility in the spring of 2012, when it will be much higher. Created with Stellarium

This time of year, the ecliptic makes a very shallow slant to the evening horizon for sky watchers in mid-northern latitudes. Venus may be a long ways to the left of the sun, but because its path is at a very low angle, it never gets far from the horizon. That’s why we have to work a little more to find it.

Every couple years, Venus puts on an evening show in the early spring sky. The next time this happens is March 2012. During spring, the ecliptic cuts the horizon at a much steeper angle. While Venus will be the same angular distance from the sun then as it is tonight, the steep slant lofts the planet high into the sky where its brilliant beacon can be seen by all.

In a sense, Venus is a bellwether for where the sun is headed next. The sun reached its highest point in the sky (farthest north) for the northern hemisphere on the June 21st summer solstice. Since then it’s been dropping to the south and sinking lower in the sky every day as we approach fall. Notice that Venus is to the east of or ahead of the sun. Where the planet is now is where the sun will be in late September as its heads southward to its lowest point in the sky called the winter solstice. In spring, Venus presages the sun’s ongoing northward climb back to its high point at the summer solstice. That’s why the planet is much higher and easier to see.

Because Venus orbits between the sun and Earth, we get to see go through phases like the moon as the angle between it, us and the sun varies during Venus 225-day long orbit. Illustration: Bob King

As I described in a recent blog, telescope users can see Venus’ phase if they look at the planet during early dusk before it gets too low for a sharp view. Its currently a “half moon” as seen from our perspective here on Earth. Over the coming weeks, Venus will appear to get closer and closer to the sun at the same time as its phase changes from half to crescent.

I wish you well in your search for this somewhat elusive planetary goddess. If you succeed in finding her, please let us know.