Date: October 31
Time: From sunset at 5:30 pm, to a few hours thereafter
Place: the southwestern sky
The planet Venus, sometimes called the Evening Star, reaches greatest elongation on October 31st. It will remain near that position for ± a couple weeks. At greatest elongation, the apparent angular distance between Venus and the Sun reaches its maximum value of about 48°. That means you could fit about 5 fist-widths (at arms length) between Venus and the Sun. So Venus will be fairly high up in the sky and easy to spot. It'll be the brightest "star" you can see.
Venus is an inferior planet: It orbits the Sun inside Earth's orbit, as does Mercury. Inferior planets cannot appear farther from the Sun (in degrees) than the angle subtended by the radius of their orbit. My animation should help you visualize the geometry. It simulates what you would see if you could "stop the Earth" and only observe the motion of Venus. Of course, the Earth rotates daily, so you'd need to piece together the motion of Venus by observing it right after sunset over a long period of time. The animation compresses 583.9 days into 30 seconds, a speed-up factor of 1.7 million. Greatest elongation occurs when Venus reaches the upper left point of its orbital motion.
Several interesting things occur at the point of greatest elongation:
- Venus will set about 3 hours after the Sun, making it easily visible in the darkening southwestern sky.
- It will be the brightest object in the sky (next to the Sun and Moon), and shine at magnitude -4.9.
- If you look through good steady binoculars (or a scope), you'll see Venus looks like a Quarter Moon.
- Many people will report seeing a UFO (this happens all the time when Venus is near greatest elongation).
Venus orbits the Sun once every 224.7 days. But since Earth is also orbiting the Sun, we're continually chasing Venus in its orbit. As a result, Venus completes the cycle shown in the animation once every 583.9 days. Note that there are two greatest elongations: one with Venus east of the Sun (as is happening now), and another with Venus west of the Sun (583.9/2 = 292 days later). To see the greatest western elongation, you'll need to get up before sunrise. But to see the greatest eastern elongation, you can conveniently observe after sunset from a favorite chair on your deck.
Over the next 292 days, the angular separation between Venus and the Sun will decrease, making it harder to observe. When it reaches conjunction with the Sun, the two will set at the same time, and Venus will transition to being the Morning Star.
Final historical note: In 1610, Galileo was the first to observe and chart how both the phase and size of Venus vary over its cycle of motion. Note how, in the animation, "full Venus" is small and "crescent Venus" is large. This is what Galileo saw. The size change, of course, is a function of distance. Galileo realized there was only one geometric arrangement that could reproduce those two changes, and that was a Heliocentric (Sun centered) arrangement. Proof that Venus (and Earth) orbited the Sun was a decisive blow to the now-defunct Geocentric Theory.
Next Week in Sky Lights ⇒ How to Find the Sun