Look out ahead!

You are hurtling at 67,000 miles an hour toward the Moon and toward Jupiter. At least, that’s briefly and roughly true tomorrow morning.

Of course the Earth doesn’t fly straight at anything; it’s moving along a curve. But the moment called First Quart (Nov. 14 15h by Universal Time) means that the Moon in its orbit around us is crossing inward ahead of us; we see it 90 degrees to the right (west) of the Sun. It’s at our future position: if it were impossibly to stop where it is, we would crash into it in something under 4 hours. It’s ahead of the front side of Earth, that is, the morning side. If you go out at sunrise, that Last Quarter Moon, shaped like a backwards D, will be in the middle of the sky, and you can tell yourself: That’s where I’m heading!

141114LastQuarSketch

And for Jupiter the moment called West Quadrature (Nov. 14 3h) means that it too appears 90 degrees to the right of the Sun, so that it is momentarily dead ahead of us. A tangent to our orbit points to Jupiter.

So, the Moon now appears on top of Jupiter in the sky, or almost. At just about the same time as the Last-Quarter instant, the Moon passes 5 degrees south of Jupiter. Not that close – 10 Moon-widths – because the Moon happens to be traveling a part of its orbit that lies south of the ecliptic, Jupiter a part of its orbit that lies slightly north of the ecliptic.

141114MoonJup

(The Moon is shown 8 times too large.)

Around sunrise, look out at nearby Moon and far-off planet. The ground under you is propelling you toward them.

The Moon, by the way, is at its apogee, its farthest from us, another coincidence (actually at Nov. 15 2h UT), so if it were to stop in its track we’d have a few more minutes to live before smashing into it.

3 thoughts on “Look out ahead!”

  1. We would reach the moon in only four hours! The Apollo astronauts took nearly three days to make it that far.

    1. Yes, Earth’s speed in orbit 67,000 mph, distance to Moon 240,000 miles. A spacecraft can’t travel in a straight line, has to depart gently from Earth’s orbit into a slightly different curving Sun-centered orbit calculated to intersect the destination body some time ahead.

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