Exercise:

At a point with latitude near 58°N,
the Last-Quarter Moon is seen rising in the north-east.

What time of day is it?
What time of year is it?

If the Moon is at Last Quarter,
it is lagging 18 hours behind the Sun -
alternatively, it is 6 hours ahead of the Sun.

The Sun crosses the meridian at noon,
so the Moon must be crossing the meridian at 6 am.

At this latitude, if it is rising in the north-east,
it must be at its most northern point,
so it will be rising about 9 hours before it crosses the meridian.
So it must be rising about 9 pm.

The Moon is at its most northern point:
the point the Sun only reaches at midsummer
(that is RA = 6h; declination = +23.4°).

And if the Moon is at Last Quarter,
it has travelled three-quarters of the way around the sky from the Sun.
So it is 270° east of the Sun -
alternatively, it is 90° west of the Sun.

So the Moon is where the Sun was 90 days ago.
So the Sun must presently be at
(summer solstice + 90 days) = autumn equinox.

Back to "The Moon".