- This is a diagram
which is not-to-scale.
- Now simple
Lunar phase questions
often seem very difficult to people.
But once you get the hang of them, they are easy.
- They are sort of analogous to a problem in algebra
with one equation and THREE VARIABLES.
You can solve for any ONE variable if you know the other TWO.
The three "variables" are:
- Lunar phase
or phase of the Moon.
- Location of Moon in the sky: eastern horizon,
eastern sky, near the celestial meridian,
western sky, western horizon,
and somewhere below the horizon.
Remember the Moon is always near the
ecliptic: i.e.,
in a day it will be carried around with the
celestial sphere
on almost the same arc on the sky as the
Sun.
- Time of solar day: e.g.,
sunrise,
noon,
sunset, and
midnight.
(Time is also the same as location on the Earth in these
problems.)
People often find this the hardest one to solve for if it is
unknown.
- The diagram is a sort of an
analog computer
that relates the THREE VARIABLES.
Just identify the two variables you know on the diagram
and the third variable is then identifiable.
- The diagram shows the
Earth-Moon
from north ecliptic pole
and this approximates the
north celestial pole (NCP) well enough
for
simple lunar phase questions.
- The Sun is so far to the right
that light rays from it
are parallel.
- The left-sides of the
Earth and
Moon are the night sides.
- The Earth rotates eastward
(i.e., counterclockwise) daily and the Moon
revolves eastward (i.e., counterclockwise) in a
lunar month.
- "You" (AKA the Alien)
locates various times of day.
"Your head" points toward the local celestial meridian.
-
Recall, the diagram is not-to-scale
In particular, the Earth
and Earth-Moon
are actually small compared to the
Earth-Moon distance,
but "you" are actually a pinprink on the Earth
which looks like an infinite plane to you.
- For example, at face-value the diagram shows "you" CANNOT see the
exact full moon
at exact sunset since it is below
your horizon at that moment.
And this is actually true for exact
lunar opposition.
You cannot see the center of the
Moon
rise at exact sunset when the center of the
Sun sets.
But the Earth is relatively small,
and Moon
and Sun have finite sizes,
and so you see the
Moon rise as the
Sun sets
or you see that so nearly as to make no difference to casual description.
- We do NOT usually worry about finicky effects due to the
finite sizes of Earth,
Moon,
and Sun when
answering simple lunar phase questions.
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