We’re looking ahead in more than one sense: the Moon will be at its Last Quarter position on October 2, and it will then be ahead of us in our travel through space. And I’m posting this on September 30 so as to be more ahead of the game than I sometimes am.
The moment of Last Quarter is Oct. 2 at 9:46 by Universal Time. This is not long after the time chosen for our picture, which is for the Central time zone of North America and for midnight between October 1 and 2. The clock time is distorted to 1 PM by “Daylight Saving Time,” still in force till November 4.
If instead you are up toward the end of the night, at 4:45 AM (or 5:45 in the Eastern zone), you can tell yourself you’re looking at the Last Quarter instant, when the Moon is 90° from the Sun. The Moon will then be much higher in the sky, with Leo rising in the east.
You can see that the Moon is close to the “Earth’s direction of travel.” At every Last Querter stage in its orbit around us, the Moon is crossing in front of us, in our common orbit around the Sun.
And we, after midnight, are traveling even faster in that direction, because we are rolling toward east – the direction from which the stars appear to be coming up, because the horizon is rolling down.
The horizon (like the celestial equator and the ecliptic) is a great circle all around the sky, so it has no curvature as we look at. The curvature each has in a flat picture depends on the projection. I now like to make the center of the projection 10 degrees below the horizon, which therefore appears to curve convexly. This is the way I sense the horizon of the sea, when I look at it each morning. And in these pictures it reinforces our sense of being on a hurtling and spinning globe.
The Moon is crossing the feet of the celestial Twins, will pass south of Castor and Pollŭ on October 3, and ascend northward through the ecliptic plane on Oct. 4.
Thanks Guy. For years I’ve made a practice of observing the Moon nightly, and following her phase, declination, ecliptic latitude, and distance from the Earth. I’m starting to notice and keep track of the Moon’s libration. The diagrams in the Astronomical Calendar helped me understand the Moon’s orbit around the Earth and our mutual orbit around the Sun. Now I need to use tabular data and mental imagery, which is harder and less fun.
Hi Guy!
Check on the third paragraph where you wrote that the Last Quarter instant is when the Moon is 180 degrees from the Sun. Shouldn’t that be 90 degrees?
Right, 90! I’ll correct it.