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Today (1/24/2020) for Boston MA timeanddate.com lists the following four times:

  • Sunrise 7:05 AM
  • Moonrise 7:06 AM
  • Moonset 4:33 PM
  • Sunset 4:47 PM

Now, if moonrise is after sunrise, I'd have thought the "new moon" conjunction already happened. But since moonset is before sunset, I'd think it hasn't happened yet.

What gives? Is this a rounding error? Usually the moon "moves" across the sky "slower" than the sun does (as in a solar eclipse where the moon moves from west to east back across the sun).

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Right now, the moon is south of the ecliptic (for those of us in the northern hemisphere). This means it encounters the horizon before the sun.

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  • $\begingroup$ Cool answer, but wait, does the Moon not also appear south of the ecliptic for those in the southern hemisphere as well? Or is it so close to the ecliptic today that location matters? Or are you just answering in a restricted way to be cautious? $\endgroup$
    – uhoh
    Jan 24, 2020 at 22:30
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    $\begingroup$ @uhoh Location does matter. If you were to check the Moonrise and Moonset times for someplace at a similar latitude to Boston, MA in the southern hemisphere, such as , Hobart, Tasmania ( timeanddate.com/moon/australia/hobart ), you'd find the Moon (5:53-21:09) rising before the Sun (6:02-20:42) and setting after it. $\endgroup$
    – notovny
    Jan 24, 2020 at 23:15
  • $\begingroup$ @notovny excellent, thanks! $\endgroup$
    – uhoh
    Jan 25, 2020 at 0:06
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The Moon does move monotonically eastward relative to the Sun. However, for observers at temperate latitudes, they rise and set at a slant, so any difference in declination affects the lengths of time they are above the horizon.

From Boston on 2020-01-24, the Moon is about 3° south of the Sun, making the time between moonrise and moonset about 28 minutes shorter than if it were at the same declination as the Sun. The Moon gains only 14 minutes of right ascension relative to the Sun in that interval, so the Moon sets a few minutes earlier even though they rose together. It appears to pass conjunction longitude sometime after setting that evening.

This sequence of Stellarium images follows the Sun and Moon from rise to set. Celestial north is up, and atmospheric refraction is neglected.

animation of Sun and Moon

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  • $\begingroup$ very cool animation! I've linked to it here $\endgroup$
    – uhoh
    Jan 25, 2020 at 3:34

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