I'm fascinated with the 2020 Great Conjunction, because (1) there is an entire word ("great conjunction") specifically reserved for when Jupiter and Saturn are in the same place in the sky, and (2) Hubble would be able to capture images of both of them in a single pointing. But this is just a dream, because the relative positions of the Earth, Jupiter, and Saturn means that the grand giants appear too close to the Sun for Hubble to target. Otherwise, a parallel exposure using HST's ACS and WFC3 cameras could have imaged both planets at the same time!
Friends have been asking me about this event, so here is some more info:
- When will it happen? The planets will be closest to each other in the sky on December 21 at 18:21 UTC (09:21 PST, thank you timeanddate.com), although this could vary by a few minutes depending on your location on the Earth. It will be daytime for half of us (including those in California), but that's OK. Jupiter and Saturn will be astoundingly close together for a few days before and after 21 December too. The best time to view them is just after sunset. They will be following the Sun down.
- But someone said it was at 13:30 UTC. The conjunction is indeed closer to 13:30 UTC (04:30 PST), but conjunction specifically means the planets are at the same east-west coordinate (right ascension) in the sky. Because their paths are at different angles, the closest approach (called the "appulse") will be a few hours later at 18:21 UTC, when Jupiter has passed slightly to the east of Saturn.
- Do I need a telescope? The great conjunction can be enjoyed without a telescope, especially if you get in the habit of taking a peek at the planets every few days leading up to it. A nice diagram here shows how Jupiter and Saturn approach over time. At conjunction, they will be closer together than any of the bright stars in the Pleiades. So depending on your eyesight, you may still be able to tell that they are two separate points of light. With binoculars, you'll be able to see Saturn's unique shape (the rings), and possibly some of the major satellites. With a telescope, you can see details on the disk of Jupiter, and the gap between Saturn and its rings... perhaps in a single frame! I am very excited to see what spectacular images the amateur astronomers manage to produce.
CREDITS: Timing and distance information comes from JPL Horizons, where geocentric observer position was selected. The simulated sky image was generated by the JPL Solar System Simulator, and the instrument footprints themselves were taken from the WFC3 Instrument Handbook. Gordy Bjoraker generated the solar system perspective view using the open-source Cosmographia visualization tool.