Happy Father’s Day all! A least to all the fathers out there.
I haven’t been sharing any astronomical thoughts for a while, as I’ve been too busy being a dad, myself. But despite my lack of writing, I’ve consistently been looking up. Enough to notice that Mars has slipped away and almost disappeared into the evening twilight, and Jupiter has started lighting up the evening skies in the east.
Last week, the news cycles were filled with typical exaggeration about “the best night to see Jupiter”, while correctly mentioning that Jupiter was at its closest approach to Earth, for this year. That point is called “Opposition”, a very Earth-centric definition that marks the time when the Sun and a given object (Jupiter, in this case) are directly opposed – directly across from each other in the sky, as viewed from Earth. As the Sun sets, Jupiter rises.
Viewed from the perspective of the entire solar system, this is the point when Earth “catches up” to Jupiter and passes between it and the Sun. It is indeed the point where the two planets are the closest.
However, what the news consistently gets wrong is suggesting that this is the only time to bother looking at Jupiter. You haven’t missed anything – Jupiter is the largest planet in the solar system, and it’s always worth a look! And despite what you may have read elsewhere, Jupiter’s largest moons (the Galilean moons – Io, Ganymede, Europa and Callisto) can always be seen with good binoculars or a small telescope. Here’s a good site to let you know which ones are which at any given time.
While we’re talking about Jupiter’s opposition, it’s a good opportunity to think of cosmic geometry in general. There’s another, more familiar object that will be directly opposite the Sun tonight, June 16. The Moon! A full Moon is one at opposition. This one happens to be called the “Honey Moon” (or the “Rose Moon”, or the “Strawberry Moon”).
As an aside – remember that neither is exactly at opposition. Jupiter was there 6 days ago on June 10, and the Moon isn’t truly full until 4:31 am EDT on the 17th. But they’re both very close!
So if we have two bodies opposite the Sun, and the orbits of both those bodies generally lie within the ecliptic (the plane of the planetary disk where all our planets were formed) – we can expect to see the Moon and Jupiter pretty much in the same place tonight.
Take a look to the east shortly after sunset, when it’s dark enough for Jupiter’s reflected light to cut through twilight and the glare of the full Moon – and you’ll see both. The Moon is about 230,000 miles away, and Jupiter about 1,670 times farther. It looks much smaller, even though its diameter is 40 times as large. Keep this in mind, you’re looking at a three-dimensional space, past the Moon to a very distant planet. It’s always helpful to imagine the relative positions in 3-D, and not think of the sky as a 2-D projection of bright objects on a black sheet.
If you’ve got a small telescope and the night is clear enough (not hazy) to cut through the Moon’s glare, take the opportunity to see if you can spot Jupiter’s moons too! It will be a challenge with the Moon washing everything out, but it’s pretty cool to imagine Earth, Moon, and then Jupiter, Io, Callisto, Ganymede, Europa all in one line, in one little slice of sky, even though we’re separated by millions of miles.
Get Out There!