The Ever-Changing Moon
The daily change in the appearance of the moon is a direct result of its orbital motion around our Earth. As the angle between the sun and the moon changes, it causes the amount of sunlight falling on the moon to also change, creating the lunar phases so familiar in the night sky.
During the approximately 14 days it takes to go from New Moon to Full Moon we see more and more of the moon, leading to the idea that it is "waxing," or building up, much like when one creates a candle; each dip in the wax adds more and more to its shape.The remaining 14, or so, days of the lunar cycle after Full Moon shows less and less of the moon so that it can be said to be "waning," or growing slimmer, until finally, reaching the New phase again, it completely disappears from our skies. The cycle then immediately repeats itself the following month.
The Four Principle Lunar Phases
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New Moon
On this day, day 0 of the start of our satellite´s journey around the earth, the moon crosses an imaginary line between the sun and Earth. Since the moon is nearly directly in front of the sun, it is lost in the glare of our star, so it cannot be seen. A few days later the moon will have moved far enough eastward along its orbit that it will be glimpsed as a thin crescent. This crescent will increase each day until the moon reaches 90° from the imaginary line between the earth and sun. |
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First Quarter
On day 7 of the moon´s trip around the earth it is 90° away from the imaginary line between the earth and sun. Since the moon is one-quarter of the way around its orbit (90° is one-quarter of 360°), it is called the First Quarter moon. From Earth, the moon appears to be half-illuminated. After tonight, the moon will appear to continue to grow with each passing night until day 14 of the moon´s trip around the earth. |
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Full Moon
Day 14 of the moon´s trip around our planet finds it 180° away from the imaginary line between the earth and sun. Here it is fully illuminated by the sun´s rays and we see the familiar Full Moon in the night sky. From this night on we will begin to see less of the moon as it continues on to the 270° point around its orbit. |
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Last Quarter
Some twenty-two days after the start of this lunar cycle, the moon has reached 270° around its orbit, which leads to this phase being called Last Quarter (270° is three-quarter of 360°). We will continue to see even less and less of the moon each night until it again crosses the imaginary line between the earth and sun and the cycle begins again with the New Moon phase. |
The Moon's Rising Time
The moon moves about 13° eastwards as seen against the background stars every day. This causes it to rise about 50 minutes later each day, but this can vary throughout the year because the moon does not move at a uniform speed and its orbit is tilted some five degrees relative to Earth´s.In the northern hemisphere the difference between successive moonrises is greatest in March when the angle of the moon´s path is steepest when compared to Earth´s horizon. In September the angle is less, so the time difference in moonrise from one day to the next is also less.
The moon´s phase can also tell us a little something about what time it will approximately rise due to its phase being directly related to the angle between it and the sun. When the moon is at First Quarter and 90° away from the sun, it will rise above the eastern horizon around noon, local time. The Full Moon, 180° away from the sun, rises as the sun is setting in the west. And the Last Quarter moon at 270° away from the sun, rises at about midnight local time.
How Fleet the Moon?
To make the complete 360° trip in its orbit around the earth in about a month, the moon must travel along fairly quickly. In fact, the moon travels at 3700 kilometers per hour, which is over twice as far as Sweden is long (about 1,574 km). And that is every hour!
Why Do We Always See the Same Side of the Moon?
Like the Earth, the moon rotates on its axis like a toy top. The main difference, however, is that Earth´s gravitational pull on the moon has slowed down its rotation so much that it is rotating at the same rate it is orbiting our planet. This leaves the same side of the moon always facing us. Well... almost. Due to a slight wobble in the moon´s motion, careful observations have shown that we actually see some 59% of its surface rather than just 50%.
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