This Sunday, Venus reaches a special position from which it is easy to estimate roughly how large the average Venus-Sun distance (RVS) is relative to the average Earth-Sun distance (RES). (I say “on average” because the Venus-Sun distance isn’t quite constant. Venus’s orbit, like that of all the planets, isn’t quite circular. But this is a small effect that we can ignore for the purpose of rough estimates.)
If you are a true diehard astronomy-geek, by all means get up at 5 or 5:30 in the morning on Sunday (or really any of the next few days) to check this directly. I can assure you (since I have been up at that time recently, due to chronic insomnia more than astronomy-geekhood) that Venus looks absolutely gorgeous against the deep blue of the pre-dawn sky. But if you have no intention on getting up that early, or clouds intervene, there’s a shortcut — on your phone.
Greatest Elongation, Near-Circular Orbits, Half-Lighting and Right Angles
On Sunday, Venus moves to a position where, from Earth’s perspective, the angle between Venus and the Sun on the sky reaches its maximum. This position is called “greatest elongation“, and it is reached twice per cycle, once in the evening sky and once in the morning. If Venus’s orbit were perfectly circular, this would also be the moment when Venus appears half-lit; as we’ve been seeing in two recent posts (1,2), that’s an effect of simple geometry:
- if Venus’s orbit were circular, then at greatest elongation, the triangle formed by Earth, Venus and the Sun would be a right angle where Venus is located, and Venus would be half-lit.
This holds for Mercury too, as it would for any near Sun-orbiting planet.
Since Venus’s orbit isn’t quite circular, this isn’t precisely true; half lit and the right angle come together, but greatest elongation is off just a few days. This is a minor detail unless you’re an astronomy-geek, and won’t keep us from getting a good estimate of the Venus-Sun distance.