Sun & Moon
The average distance between the earth and the moon is about 240,000 miles. The orbit of the moon around the earth is an ellipse (a circle with two centers), so the distance is not constant. The point in orbit where the moon is farthest from the earth is called the apogee. This distance is about 254,000 miles. The point in orbit where the moon is closest to the earth is called the perigee. This distance is 227,000 miles.
If viewed from above the north pole, the earth rotates counterclockwise on its axis and orbits counterclockwise around the sun. The moon also rotates counterclockwise and orbits counterclockwise around the earth. However, they travel and rotate at much different speeds. The earth obits the sun at about 66,620 mi/hr while the moon obits the earth at the speed of only about 36 mi/hr.
The moon is about 1/4 the size of earth and has about 1/6 the gravity. Earth’s gravity holds the moon in orbit and controls the rate at which it spins.
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Because the rotation and revolution of the moon take the same amount of time, 27.3 days, observers on the earth always see the same side of the moon. Astronomers call this captured rotation. Because of captured rotation, there is a side of the moon which no one on earth ever sees. Often called the far side of the moon or the dark side of the moon, this region was first seen by a Russian spacecraft in 1959.
The moon shines by reflecting sunlight from its surface. Half of the moon is lighted while the other half is dark. As the moon revolves around the earth, you see different portions of its lighted side, causing its appearance to change.
Moon phases are the changing appearances of the moon as seen from the earth. The phase you see depends on the relative positions of the moon, Earth, and sun. One moon phase lasts 29 calendar days, 12 hours, 44 minutes, and 29 seconds, or 29.5 days.
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A moon phase or month begins with a new moon. A new moon occurs when the lighted half of the moon is facing the sun and the unlighted side faces the earth. The moon is in the sky, but cannot be seen because there is no light reflecting from it onto us. As the moon continues its orbit around the earth, part of its lighted half becomes visible.
When the visible portion of the moon is increasing, the moon is in its waxing phases. When a sliver of the moon becomes visible from the earth, the moon enters the waxing crescent phase.
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When the moon has moved through one quarter of its orbit after the new moon, half of the lighted side is facing the earth. This is called the first quarter phase. When the visible portion on the moon is larger than a semi-circle and is still increasing, the moon is in the waxing-gibbous phase.
At a full moon the earth is between the sun and the moon. The entire lighted side of the moon is seen from the earth. After the full moon phase, the portion of the moon visible from the earth deceases. This is called the waning phases of the cycle. The moon moves through the waning gibbous phase, last quarter phase or third quarter phase, and waning crescent phase until none of the lighted portion faces the earth, and a new moon appears to start another cycle.
The phrase “once in a blue moon” means not very often. It comes from the fact that every 2.5 years one month will have a second full moon called a blue moon. This happens because each month has more time than it takes to complete one moon cycle.
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Although the moon revolves around the earth in 27.3 days, a longer period of time is needed for it to go through a complete cycle of phases. The period from one new moon to the next is 29.5 days. This 2.2 day difference is due to the orbiting of the earth-moon system around the sun. In the 27.3 days in which the moon orbits the earth, the two bodies move slightly farther along in their orbit around the sun. Therefore, the moon must go a little farther to get directly between the earth and the sun. This is the position of the moon in the new moon phase. About 2.2 days are needed for the moon to travel this extra distance and catch up with the earth.
When something is eclipsed, it is blocked, or its view is obscured. An eclipse occurs when the earth or moon temporarily block sunlight from reaching the other causing a shadow on one. They can only occur when the sun, the moon, and earth are perfectly lined up.
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If the plane of rotation of the earth and the moon were lined up exactly, solar and lunar eclipses would happen every month. However, the orbit of the moon is tilted at a 5° angle to the plane of the earth and sun.
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A solar eclipse occurs when the moon moves directly between the sun and the earth blocking sunlight and casting a shadow on part of the earth. Only the outer portion of the sun’s atmosphere is visible at this time. Only a small area of the earth experiences a total solar eclipse. A person must be within the shadow region to see the eclipse, so they are difficult to see. A total solar eclipse occurs up to twice a year.
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A lunar eclipse occurs when the earth moves in between the moon and sun. The earth’s shadow falls on the moon. The moon may appear dark red. Lunar eclipses also occur twice a year, but they are much easier to see than a solar eclipse because anyone on the night time side of earth can see the lunar eclipse. Lunar eclipses may last several hours, much longer than a solar eclipse, because the earth casts a much larger shadow than the moon.
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The daily changes in the level of the ocean surface are called tides. Tides are caused by the gravitational pull of the moon and the sun, but primarily the moon since it is closer. The moon’s pull is strongest on the part of the earth directly facing the moon. The moon’s gravity actually pulls the earth’s water as it orbits the earth causing a slight bulge. The water on the opposite side of the earth also bulges. This is called a high tide. The area 90° to the tides, where the water is drawn away from, is called low tide. There are two high tides and two low tides on earth each day.
Since the earth rotates, different parts of the water face the moon daily which causes the tides to keep changing. The difference between high tide and low tide is called a tidal range.
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Lakes also have tides, but the changes are very small. Lake Superior has a tidal range of only 2 inches. If the earth and moon did not move, tides would always occur in the same locations. Since the earth rotates from west to east, tidal bulges appear to move westward around the earth. Because there are two tidal bulges, high tides and low tides are less than a day apart.
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The sun also affects the tides. When the sun, moon, and Earth are in alignment, there is a greater gravitational pull. High tides are higher, and low tides are lower. This alignment is called spring tide. It occurs every 14 calendar days, or during the full moon and new moon.
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When the sun, moon, and Earth are at a 90° angle, gravitational forces work against each other so the tides are lower. This is called neap tide. It occurs half way between the spring tides during first quarter moons and last quarter moons.
One low tide-high tide cycle takes 12 hours and 25 minutes. A daily cycle of two highs and lows, a tidal day, takes 24 hours and 50 minutes. A tidal day is the time period between two successive passes of the moon over a point on the earth.
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While the earth turns, the moon is also moving eastward along its orbit about the earth. Like the sun, the moon appears to rise in the east and set in the west. The earth’s rotation brings the moon into view every day. After 24 hours, the earth point that began directly under the moon is no longer directly under the moon. The earth must turn another 12°, requiring an additional 50 minutes, to bring the starting point on earth back in line with the moon. So, as the moon revolves around the earth, the earth must rotate through more than one complete rotation to catch up with the moon. As a result, the moon rises and sets a little later each day. The earth must rotate 24 hours and 50 minutes to bring the moon back into the same position. So the moon rises 50 minutes later each day. This explains why tides arrive at any location about one hour later each day.
The earth’s rotation speed is not constant. Right now the earth’s rotation is slowing down. This is caused by the friction of the ocean tides rubbing against the continents. On average the earth’s rotation is slowing 1/500th second each day. Every 500 days, we lose one second.
Full moon falls on June 23, 2013 at 11:32 UTC (6:32 a.m. CDT in the U.S.). Thus, for many, the moon appears about as full in the June 22 evening sky as it does on the evening of June 23. This full moon is not only the closest and largest full moon of the year. It also presents the moon’s closest encounter with Earth for all of 2013. The moon will not be so close again until August, 2014. In other words, it’s not just a supermoon. It’s the closest supermoon of 2013.
At United States’ time zones, the moon will turn full on June 23 at 7:32 a.m. EDT, 6:32 a.m. CDT, 5:32 a.m. MDT and 4:32 a.m. PDT.
June 23 supermoon helps you imagine spacecraft en route to Pluto
We astronomers call this sort of close full moon a perigee full moon. The word perigee describes the moon’s closest point to Earth for a given month. Two years ago, when the closest and largest full moon fell on March 19, 2011, many used the term supermoon, which we’d never heard before. Last year, we heard this term again to describe the year’s closest full moon on May 6, 2012. Now the term supermoon is being used a lot. Last month’s full moon – May 24-25, 2013 – was also a supermoon. But the June full moon is even more super! In other words, the time of full moon falls even closer to the time of perigee, the moon’s closest point to Earth. The crest of the moon’s full phase in June 2013, and perigee, fall within an hour of each other.
What does supermoon mean exactly? And how special is the June 23, 2013 supermoon? Follow the links below to find out.
What is a supermoon?
How super is this supermoon?
How often is moon both full and closest to Earth?
Will the tides be higher than usual?
Your best photos: May 2013 supermoon
Does a supermoon have a super effect on us?
Big sun-diving Comet ISON might be spectacular in late 2013
View larger. | Astronomers say it’s tough to notice the difference in size between a supermoon and any other full moon. But photographs show it. Check out this size comparison from our friend Alec Jones in the UK.
The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Note the size difference. Image via Marco Langbroek, the Netherlands, via Wikimedia Commons.
What is a supermoon? The word supermoon didn’t come from astronomy. Instead, it came from astrology. Astrologer Richard Nolle of the website astropro.com takes credit for coining the term supermoon. In 1979, he defined it as:
…a new or full moon which occurs with the moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee). In short, Earth, moon and sun are all in a line, with moon in its nearest approach to Earth.
By this definition, according to Nolle:
There are 4-6 supermoons a year on average.
That doesn’t sound very special, does it? In fact, the June 2013 full moon lines up much more closely with perigee – the moon’s closest point to Earth – than Nolle’s original definition. According to Guy Ottewell’s Astronomical Calendar 2013, the 2013 June full moon falls only 22 minutes after the moon reaches perigee, the moon’s closest point to Earth for this month and year. At perigee, the moon lies only 356,991 kilometers (221,824 miles) away. Two weeks later, on July 7, the moon will swing out to apogee – its farthest point for the month and year – at 406,490 kilometers (252,581 miles) distant.