As a kid I would always wonder why the size of moon keeps on decreasing and finally disappear. And this rotation of slowly disappearing of moon became a routine. I thought that it was some magic happening in the sky. There is a myth that only one part of moon is seen and the other part is always dark. But the fact is that only one side of moon is visible to one part of the world while the other part is not see. This is because the moon rotates at the same speed as the earth’s rotation.
The phases of moon is how it looks from the earth. Moon is always illuminated by sun. Moon orbits earth at 382400 km on an average. One lunar takes 27.
There are 8 phases of moon appearing every month.
- New moon: On a new moon day there will be no moon seen in the sky. The sky appears fully dark. Sun, moon and earth are lined in the same line. So the moon is fully shadowed. That is the reason it appears dark. The condition will be something like solar eclipse. On a new moon the condition is such that the moon rise and sets along with the sun rise and setting. So the moon and sun are in parallel actions. This situation cause moon to appear dark when viewed from the earth surface.
2. Crescent moon: The slight appearance of moon after the new moon is the crescent moon. So it’s the first silver line of moon visible. This waxing of moon may appear for few days. The moon rise and set is several hours behind the sun. So the moon tends to follow the sun. In the southern hemisphere the illuminated edge of the moon on left and it is the reverse for the northern hemisphere. The crescent will increase day to day.
- First quarter: It’s a more lit up than crescent moon. It’s the next portion being seen after the crescent moon. But it is not half-moon. The half portion is commonly known though it is not so. The moon appears to illuminated half with reference to new moon, this is the reason it is popularly known to be half-moon. Technically it is the first quarter. This appears nearly one week after the new moon.
4.Waxing gibbous moon: The gibbous moon is more illuminated that the first quarter. It appears to be lit up more than half but less than full. It has repositioned in its orbit. At this period the moon is relatively far away from the sun. The gibbous moon rise in between noon and sunset. And sets somewhere in the midnight. Since it is far from the sun there are no sun glares hiding the moon. The visibility of moon increases day by day.
5.Full moon: The entire moon surface is visible from the earth. The moon and earth are in opposite from the sun. Lunar eclipse occurs in such conditions. So moon sun and are setup in same line. Earth is in between the moon and sun. The moon rise when the sun sets and sets when sun rises. It has opposite actions. New moon and full moon are totally opposite. The full moon appearance is two week after the new moon.
6.Waning gibbous moon: After the full moon, the visibility of moon gradually perishes. This is supposed to be the second half cycle of the lunar phases. It appears in the west. It is similar to waxing gibbous moon. Illuminated more than half moon and less than full moon.
7.Last quarter moon: The moon is in its orbit. Earth is at right angle to the sun. The last quarter moon is at the three quarter stage from one new moon to the other new moon. Eventually the moon comes closer to the sun. Thus there is reduction in the visibility of the moon. It rises in the midnight. The moon along with earth is orbiting the sun.
8. Waning crescent moon: The size of the illuminated moon is shrinking day by day. It is the last stage before the new moon. The total period from one new moon to another new moon is calculated to be a month. The moon is again in line with sun and earth again. The sun glares darkens the moon and obstructs its visibility.