If you can understand when the moon is waning or waxing, you can determine which phase it is in, what its position is relative to the Earth and the Sun and how it affects the tides. It is also important to know where it will arise according to the different phases, in case you want to observe it on a particular night. There are a couple of ways to gauge whether you are looking at a waning or waxing moon; although certain details change based on your position on the planet, the method does not vary.
Steps
Part 1 of 3: Understanding the Moon Phases
Step 1. Learn the names of the phases
The moon rotates around the Earth and, during this movement, its surface is illuminated at different angles. Our satellite does not have its own light, but reflects that of the Sun. When the moon passes from new to full, to return to new, it goes through various transition phases, which are identifiable by the curvature of the "illuminated segment". The phases of the moon are:
- new Moon
- Crescent moon
- First quarter
- Growing gibbous
- full moon
- Waning gibbous
- Last quarter
- falling moon
- new Moon
Step 2. Learn the meaning of each stage
The moon always follows the same trajectory around the Earth every month, so it constantly goes through the same phases. These are determined by the perspective from which man observes the illuminated portion, which changes according to the relative position of the Sun, Earth and Moon. Remember that half of the Moon is always illuminated by the Sun, but it is our point of view (from the Earth) that determines the phase we can observe.
- When the moon is new, its position is between the Earth and the Sun, so we cannot see its illuminated face. In this phase, the illuminated side is completely turned towards the Sun and we "see" only the face in shadow.
- In the first quarter we can see half of the face illuminated and half of the face in shadow. This situation is repeated in the last quarter, but the sides we observe are opposite.
- When the moon appears full to us, we are able to see the fully illuminated half, while the "dark" side faces space.
- Once it reaches the full moon position, the satellite continues its revolution motion around the Earth and the Sun, reaching the new moon phase.
- To complete a revolution around our planet, the moon takes just over 27 days. However, a full lunar month (from one new moon to another) is 29.5 days, because this is the time it takes for the satellite to return to the same position between the Earth and the Sun.
Step 3. Learn why the moon waxes and wanes
As the satellite transitions from the new moon phase to the full moon phase, we see an ever larger wedge than the illuminated half and we refer to this transition as "growth". On the other hand, when the moon passes from full to new, the visible segment of the illuminated portion becomes smaller and smaller, so we assume that it is "waning".
The phases are always identical, even if the moon appears in different points and orientations of the sky, so you can always identify them by observing particular details
Part 2 of 3: Determine the Moon Phases in the Northern Hemisphere
Step 1. The moon waxes and wanes from right to left
During the various phases different portions of the Moon are illuminated. In the northern hemisphere the luminous part grows in size with an apparent movement from right to left until it reaches the full phase, and then decreases always from right to left.
- The waxing moon is illuminated from the right and the waning one from the left.
- Keep your right hand raised with the thumb up, palm towards the sky. The thumb, with the fingers, creates a sort of reverse C. If the moon matches this curve, it is waxing. If you do the same thing with your left hand and the moon matches the C, it is waning.
Step 2. Remember diagram D, O, C
Since the moon always follows the same lighting pattern, you can use the letters D, O and C to tell if it is waning or waxing. In the first quarter the illuminated segment looks like a D, in the full phase the moon looks like a letter O and in the last quarter the segment has the shape of a C.
- The crescent moon is shaped like an inverted C
- The D-shaped gibbous moon is waxing
- The inverted D-shaped gibbous moon is waning
- The waning moon is shaped like a C.
Step 3. Learn when the moon rises and sets
Our satellite does not appear in the sky all the time at the same time, because the time varies according to the phase. This means that you can use the rising hour and the setting hour to figure out if it is waning or rising.
- The new moon is not visible both because the face facing the Earth is not illuminated, and because it rises and sets together with the Sun.
- When the waxing moon enters the first quarter phase, it rises in the morning, reaches its maximum height around the setting of the Sun and disappears from our sight at midnight.
- The full moon rises at sunset and disappears at dawn.
- During the last quarter, the moon rises at midnight and sets in the morning.
Part 3 of 3: Determine the Moon Phases in the Southern Hemisphere
Step 1. Learn which parts of the moon are illuminated during the waxing and waning phases
Unlike in the northern hemisphere, in the southern hemisphere the moon appears to wax from left to right, becomes full and wanes from left to right.
- The moon illuminated from the left is waxing, while it is waning when illuminated from the right.
- Keep your right hand raised with your thumb out and palm facing the sky. The thumb and fingers create a curve to form an inverted C. If the moon fits this curve, it is waxing. If you do the same thing with your left hand and the Moon fits into C, it is waxing.
Step 2. Remember the sequence C, O, D
Our satellite always follows the same phases also in the southern hemisphere, but assumes shapes that resemble the letters of the alphabet with an opposite sequence.
- The crescent moon is shaped like a C.
- The crescent gibbous moon has the shape of an inverted D.
- The full moon looks like an O.
- When it is waning gibbous it looks like a D.
- A waning moon is shaped like an inverted C.
Step 3. Learn when the moon rises and sets
Even though it is illuminated from the opposite side to that of the northern hemisphere, our satellite rises and sets at the same times according to the phases.
- In the first quarter, the moon rises in the morning and sets around midnight.
- The full moon rises at sunset and disappears at dawn.
- The moon in the last quarter rises at midnight and sets in the morning.
