Objectives

  • distinguish between the different phases of the Moon
  • investigate how the Moon changes phases throughout a month
  • predict where to locate the Moon in one of its phases

As the Earth rotates, the Moon also revolves around the planet. Because of this, not only can the Moon be found in different locations in the sky depending on the day, but the Moon also looks different to an observer on the Earth. Over the course of the Moon’s rotation period (the amount of time it takes the Moon to go around the Earth once, equal to about 27.3 days) the Earth-Moon-Sun geometry changes so that there is always a different amount of sunlight shining on the Moon. This is what gives us our phases.


waningCrescent.jpg
A waning crescent Moon


Perhaps the easiest phases of the Moon to visualize are the New Moon phase and the Full Moon phase. A New Moon occurs when the Moon comes between the Earth and the Sun. As a result, the Sun only lights up the side of the Moon away from us, and the Moon appears dark, hard to see. Occasionally, when the Sun and Moon line up perfectly, we see a solar eclipse (see Unit 4, Lesson 2).




The Full Moon is the exact opposite. When the Moon is on the opposite side of the Earth from the Sun, sunlight reflects off the entire surface of the Moon facing the Earth, and we see a round, lit up Moon. Full Moons are often so bright that many astronomical ground observations cannot be made during this phase, as the light from the Moon washes out the fainter stars. Occasionally the Earth comes directly in between the Moon and the Sun, creating conditions for a lunar eclipse (see Unit 4, Lesson 2).




A full lunar month goes from New Moon to New Moon and takes approximately 29.5 days. This is slightly longer than the Moon’s rotational period, because we have to keep in mind that the Earth is also moving around the Sun, constantly changing the Earth-Moon-Sun geometry.

lunarMonth.jpg
Even though it takes the Moon 27.3 days to go around the Earth once,
by the time it gets back to its original position, the position of the Sun
has changed, slightly altering the visible phase of the Moon. Because
of this, a lunar month takes slightly longer: 29.5 days.

Exactly between the New Moon and the Full Moon, comes two more common phases: the First Quarter Moon and the Last Quarter Moon. The First Quarter Moon occurs during the first half of the lunar month, when the Moon-Earth-Sun system forms a 90? angle. At this time, rays from the Sun hit one half of the Moon’s surface, of which we can only see a half, leaving us with a quarter - half of the half which is being lit. Likewise, the Last Quarter Moon occurs during the last half of the lunar month, when the Moon-Earth-Sun system forms another 90? angle. During this phase, we again only see a quarter of the Moon.

phasesMoon.jpg
The New, First Quarter, Full and Last Quarter phases of the Moon,
as seen from far above the Earth’s North pole, and from the surface
of the Earth.



It should be remembered that the Sun’s light always hits half of the surface of the Moon, just like it always hits half of the surface of the Earth. The phases occur because of our angle to the Moon.





As the Moon travels around the Earth, we see “in between” phases of the Moon as it goes from one of the four main phases to another. Whenever the Moon is in between the New Moon phase and one of the Quarter phases, it is said to be a crescent Moon (due to it’s croissant-like appearance!). When the Moon is in between one of the Quarter phases and the Full Moon, it is said to be a gibbous Moon. This rarely-seen word comes to us from the Latin gibbus, meaning hump, or humpbacked.





Finally, as the Moon goes from New Moon to Full Moon during the first half of the lunar month, it is known as a waxing Moon, and when it returns from Full Moon to New Moon during the latter half of the lunar month, it is known as a waning Moon. The terms waxing and waning are also sometimes used in reference to the tides on the Earth (see Unit 4, Lesson 1) – the tide coming in and the tide going out, respectively.




In summary, the Moon goes through phases in the following order:


New Moon - waxing crescent Moon - First Quarter Moon - waxing gibbous Moon - Full Moon - waning gibbous Moon - Last Quarter Moon - waning crescent Moon - New Moon

phasesMoon2.jpg
All phases of the Moon, as seen from far above the Earth’s North
Pole and from the surface of the Earth


Using a telescope, we can see similar phases on planets closer to the Sun than us – venus and Mercury. When Galileo first saw the phases of Venus through a telescope, his observations helped contribute to a Sun-centred solar system, as opposed to an Earth-centred one which was favoured at the time.





Though the Moon goes through all of these phases many times a year, certain, special phases stand out from the others. A Blue Moon is the second Full Moon within one calendar month (such as, a Full Moon on January 2, and then another one on January 31). While the expression “once in a blue Moon” denotes a rare activity, Blue Moons actually occur roughly every 3 years. The Full Moon that occurs closest to the autumnal equinox (see Unit 1, Lesson 2) is known as the Harvest Moon, as the light of that particular Full Moon was useful to farmers working late into the night to finish the harvest before it got cold. Likewise, the following Full Moon is known as the Hunter’s Moon, due to its same contribution to hunting at night.





The phases of the moon are very apparent and consistent for observations. That’s why many cultures have used the moon to keep track of time, employing the lunar calendar, based on the lunar month, or lunation, of 29.53 days. Because this month is shorter than the modern solar month, 12 lunations (or 354.37 days) is not able to match the 365.25 days of a solar year. The seasons therefore become out of synch with the lunar months. A lunisolar calendar brings the lunar and solar calendars back into synch with a leap month that occurs every two or three years to rectify the problem.





The Islamic calendar is a modern example of a lunar calendar that repeats itself every 360 lunar months with a correction of one day. Many other cultures such as the Chinese, Korean, Hebrew, Tamil and Hindu cultures have adapted their lunar calendars to lunisolar calendars to some extent. A lunisolar calendar is still used by Christians to determine the date of Easter, as it is falls on the Sunday after the first full moon after March 21, the vernal equinox.

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