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Does the Moon Rise and Set? and Answers to other Pressing Questions about the Moon
A fisherman living on the Atlantic seaboard may observe that high tide occurs twice a day. Therefore, he may conclude that the Moon rises twice a day. Would he be correct? Well, it's an intelligent observation, but it is not correct.
Simply put, a moonrise occurs in one complete rotation of the Earth plus 12°. It takes 24 hours for the rotation plus 50 minutes for the 12°. So now we know that a moonrise occurs every 24 hours and 50 minutes and not twice a day.
The Moon revolves around the Earth approximately every 29.5 days. But since the Earth is always rotating, and the Moon is revolving around it in the same direction as that rotation, the Moon only moves 12° over Earth's surface.
But when does the moonrise take place? A little knowledge concerning the phases of the Moon will help pinpoint when a moonrise occurs:
The New Moon (that is, when we cannot see it, also called the Dark Moon) rises at sunrise.
The First Quarter Moon rises at noon, when the Sun is directly above.
The Full Moon rises at sunset.
The Third Quarter Moon (or Last Quarter) rises at midnight.
There are seven days between each of these phases of the Moon. For each day following the above moon risings, the Moon will rise about 50 minutes later than the previous day.
Where do the tides fit in? The highest tides occur during the New Moon, First Quarter Moon, Full Moon and Last Quarter Moon.
During the New and Full Moons, the Moon, the Earth, and the Sun are lined up, and Spring Tides occur.
During the New Moon, the Moon's position is between the Earth and Sun. During the Full Moon, the Moon is on the opposite side of the Earth from the Sun, the Earth taking the position between.
Spring Tides are so named because the tides "spring" out from the Earth creating a tidal bulge on each side of the globe. Also, the tides "spring" out and back again, causing periods of very high tides and very low tides.
During the New and Full Moon phases, the Moon pulls Earth's waters toward itself, creating high tides on that side of the Earth. Tides on the opposite side of the Earth are almost equally as high since the Moon also pulls the Earth away from the water on that side. The gravitational pull of the Sun also plays a role; its pull is about half that of the Moon, due to the Sun's greater distance from the Earth.
Neap Tides are the high tides that occur during the First Quarter and Last Quarter phases of the Moon. Neap tides are not as high as spring tides, and tidal range is minimal.
During the First Quarter and Last Quarter phases of the Moon, the Sun and Moon are at right angles to each other. The Earth would be the 90° corner if an imaginary triangle were drawn to connect the three orbs. Due to their positions at right angles, the pull of the Sun and Moon cancel each other out to some degree, thus making the tides less spectacular.
So why do we see two tides each day? It has a lot to do with where you live.
One tidal bulge always stays under the Moon, and the other tidal bulge always stays on the opposite side of the Earth. As the Earth rotates, a high tide occurs about twice a day on most parts of the ocean.
The two high tides at a given place do not usually rise equally high because the centers of the two tidal bulges usually lie on opposite sides of the equator and not on the equator. This is because the Moon is usually located either north or south of the equator.
Atlantic Ocean. On Atlantic coasts, we observe two tides each day because the Atlantic Ocean has tides that flow and ebb twice a day. From its lowest point, the water rises for about 6 hours until it reaches high tide. This is called Flow Current or Flood Current. After the sixth hour of rising water, Ebb Current occurs and the water level falls for the next 6 hours until it reaches low tide.
Pacific Islands. The Pacific Islands experience Mixed Tides, that is, two high tides per day with little ebb between; and then, a very low tide.
Saint Michael, Alaska and Gulf of Mexico. In these two locations, there is only one high tide and one low tide daily.
Mediterranean Sea. Has very little tide.
Why all the variations? The frequency of tides and their range depend on the depth of the water, the width of the water body and the shape of the sea-bed beneath.
The average tidal range is three to ten feet around the globe. In V-shaped bays, like the Bay of Fundy, the tide may rise and fall 30 to 50 feet. In broad bays, the tides may rise and fall by one foot or less.
When does moonset occur? The Moon sets 12 hours and 25 minutes after it rises, or halfway between each moonrise.
What is a Blue Moon?
Is a Blue Moon really so rare or even impossible? Basically there are three kinds of Blue Moons.
- The Moon can actually appear blue in color, but only if there has been a major volcanic eruption. The resulting particles of ash cause the Moon to appear blue. Otherwise, the color of the Moon as seen from Earth is either yellow or white.
- Third in a season containing four. When there are four Full Moons in one season--most seasons contain three--the third is called a Blue Moon. This is the correct definition of a Blue Moon. On the average, this event occurs every 2.7 years, sometimes more frequently. There was a Blue Moon of this type on August 20, 2013. The next one will occur in 2015.
- The third type of Blue Moon is the one most commonly named, but it is not correct. In 1946, "Sky and Telescope" magazine mistakenly defined a Blue Moon as a second Full Moon in one month, most months having only one Full Moon. This definition, being in print and possibly easier to understand, mainstreamed into public usage. We have seen this phenomenon twice in 1999, once in January and once in March. There was no Full Moon in February of 1999. Because of February's shorter length of days it occasionally does not contain a Full Moon and 2 Full Moons will occur in other months.
So why do we use the expression 'once in a Blue Moon' to mean rarity or impossibility?
The use of the expression to denote an absurdity or an impossibility was first coined in the 16th Century by Cardinal Wolsey, the advisor of Henry VIII. He wrote of his adversaries as those 'who would have you believe the Moon is blue'.
Over the centuries the expression evolved to mean 'a very long time'. In a book about working class London, written in Great Britain in 1821, a man quoted as saying, 'I haven't seen you this Blue Moon'. The popularity of the phrase is believed to have started in Great Britain.
Miscellaneous but Fun Moon Facts
- It's going away! When the Moon was first formed it was 14,000 miles (22,530 km) away from the Earth. It is now 280,000 miles (450,000 km) away. Why so? Each year, the Moon is propelled 3.8 cm higher in its orbit by to the rotational energy of the Earth.
- It is slowing us down. As a result of the Moon's gravitational pull on the Earth and on its waters, the Earth's rotation slows down at a rate of about 1.5 milliseconds per year.
- Dark Side of the Moon. Over time, the Earth's gravitational effect has slowed the Moon's rotation. Gradually, the Moon's rate of rotation has slowed enough to match the time it takes to orbit the Earth, until finally, only one side of the Moon faces Earth. It is not really always dark, it is just the side we never see from Earth. Other moons around other planets have behaved similarly.
- Moonquakes? As a result of Earth's gravitational pull on the Moon, small moonquakes occur miles beneath the Moon's surface causing cracks on the surface of the Moon through which gas escapes.