Caption: The ONE DIAGRAM---which you can reproduce for yourself whenever you need it---that allows you to answer all simple lunar phase questions.

One diagram to bring them all and in the darkness bind them
---One Ring, etc.

1. This is a diagram which is not-to-scale.

2. Now simple Lunar phase questions often seem very difficult to people.

   But once you get the hang of them, they are easy.

3. 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:

   1. Lunar phase or phase of the Moon.

   2. 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.

   3. 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.

4. 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.

5. 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.

6. The Sun is so far to the right that light rays from it are parallel.

7. The left-sides of the Earth and Moon are the night sides.

8. The Earth rotates eastward (i.e., counterclockwise) daily and the Moon revolves eastward (i.e., counterclockwise) in a lunar month.

9. "You" (AKA the Alien) locates various times of day.

   "Your head" points toward the local celestial meridian.

10. 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.

11. 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.

12. We do NOT usually worry about finicky effects due to the finite sizes of Earth, Moon, and Sun when answering simple lunar phase questions.