Despite its great elongation from the sun, Venus remains low in the southwestern sky during evening twilight for observers in mid-northern latitudes. Credit: Bob King
Demure Venus. Here’s a planet, brightest and arguably the most beautiful in the sky, that’s been hiding for months. It’s out there alright, but you better have a clear, open view to the west. For skywatchers living at mid-northern latitudes, which includes much of the U.S., Canada and Europe, Venus stands only about 10 degrees high (one fist held at arm’s length) in the southwestern sky a half hour after sunset.
Today it reaches its greatest elongation from the sun in the evening sky of 47 degrees - as far away as it can possibly get. But you’d hardly know it. The planet of love and beauty has been hiding near the horizon for months. It’s not Venus’ fault.
Because the angle of the ecliptic to the evening horizon is shallow this time of year, most of Venus’s distance from the sun is nearly horizontal to the horizon with little of it going toward height or altitude. Maps: Stellarium
In the fall of the year, the path taken by the planets, sun and moon, called the ecliptic, makes a very shallow angle to the western horizon. Even though Venus is far from the sun, most of that distance is along the horizontal (to the east) with only a smidge in the vertical. After sunset, the rotating Earth makes quick work of putting the planet to bed.
In the spring, the ecliptic meets the horizon at a much steeper angle. When Venus reaches great elongation in April-May, most of that 47 degrees is straight up and vertical to the sun. Venus then shines in a dark sky for hours after twilight’s end, blazing like a sapphire.
Further south in Hawaii or Jamaica, the planetary highway (ecliptic) is tilted up more steeply. Venus follows suit and stands high above the horizon and sets later than from locations further north.
Of course I’m biased because I’m in my mid-forties – latitude that is. If you’re reading this from Jamaica or Hawaii you might be scratching your head. From those sunny locales, Venus shines in a pitch black sky and sets considerably later than in Cincinnati. How so?
Venus looks just like the first quarter moon in a small telescope. Look during twilight for the best view.
As you travel south along Earth’s curving surface, the angle of the ecliptic tilts ever upward, taking any planets along with it. Keep flying south beyond Jamaica to Lima, Peru and the ecliptic intersects the western horizon vertically at dusk.
All of those 47 degrees go toward lofting Venus high in the sky. The planet will make a stunning sight this coming month reflecting from the placid waters of the Caribbean and points south.
Venus reaches greatest elongation east (left) of the sun today on Halloween when its apparent distance from the sun is at maximum. In the coming weeks, Venus distance from the sun will decrease and its phase will shrink to a crescent as it slips between Earth and sun. Illustration: Bob King
Venus’s phase changes as it revolves around the sun interior to Earth’s orbit. Greatest elongation is a special time because the sun, Earth and Venus make a 90 degree angle to each other. From our perspective, Venus appears exactly 50 percent illuminated, looking just like a half moon sans pockmarks and spots. A perpetual cloud blanket covers the planet making it shine a fierce white.The best time for viewing the planet is during twilight, when sky light tames its terrific glare.
In the coming weeks, Venus will rise a bit higher in the southwest as the ecliptic angle increase slightly and the sun continues to set earlier. Come late November, watch for it to drop even lower as the angle it makes to Earth and sun narrows.
I should point out that while Venus hovers its highest in the sky for the next month, it’s not at its most brilliant. That occurs on Dec. 10 when it will shine at magnitude -4.6, slightly brighter than it is now. Even though the planet’s phase will only be a crescent then, its proximity to Earth more than compensates for its skinny profile.