The Ocean's Tides Explained

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The Ocean's Tides Explained

The alternating pattern of rising and falling sea level with respect to land is what we know as thetides. What causes this "motion of the ocean"? In one word, gravity. Specifically, the gravitational forces of the Sun and Moon. 

The key to understanding how the tides work is understanding the relationship between the motion of our planet and the Moon and Sun. As the Earth spins on its own axis, ocean water is kept at equal levels around the planet by the Earth's gravity pulling inward and centrifugal force pushing outward. 

However, the Moon's gravitational forces are strong enough to disrupt this balance by accelerating the water towards the Moon. This causes the water to 'bulge.' As the Moon orbits our planet and as the Earth rotates, the bulge also moves. The areas of the Earth where the bulging occurs experience high tide, and the other areas are subject to a low tide

Water on the opposite side of Earth facing away from the Moon also bulges outward (high tide), but for a different and interesting reason: in reality, the Moon and the Earth revolve together around a common gravitational center between them, or center of mass. Here's a rough but helpful analogy: picture yourself swinging a heavy object attached to a rope around your body as you rotate. You have to lean back to compensate, which puts the center of mass between you and the object. With the Earth-Moon system, gravity is like a rope that pulls or keeps the two bodies together, and centrifugal force is what keeps them apart. Because the centrifugal force is greater than the Moon's gravitational pull, ocean water on the opposite side of the Earth bulges outward.

The same forces are at play as the Earth revolves around the Sun. The Sun's gravity pulls ocean water toward the Sun, but at the same time, the centrifugal force of the combined Earth-Sun revolution causes water on the opposite side of Earth to bulge away from the Sun. However, the effect is smaller than the Moon, even given the greater mass of the Sun (greater mass means greater gravitational force). Why? Simply because The Sun is so far away — over 380 times farther away from the Earth than the Moon. 

Because the tides are influenced by both the Moon and the Sun, it's easy to see that when the Sun lines up with the Moon and the Earth, as during a New Moon or Full Moon (a configuration also called "syzygy"), the tidal effect is increased. These are known as spring tides, named not for the season, but for the fact that the water "springs" higher than normal. 

On the other hand, if the Sun and the Moon are 90 degrees apart in relation to an observer on Earth as during the First Quarter Moon or Third Quarter Moon (sometimes called half moons), then high tides are not as high as they normally would be. This is because despite its greater distance, the Sun's mass allows it to exert enough gravitational force on the oceans that it can negate some of the effects of the Moon's pull. This phenomenon of lower high tides is called aneap tide

The height of the tides can also vary during the course of a month because the Moon is not always the same distance from the Earth. As the Moon's orbit brings it in closer proximity to our planet (closest distance within a moon cycle is called perigee), its gravitational forces can increase by almost 50%, and this stronger force leads to high tides. Likewise, when the Moon is farther away from the Earth (furthest distance is called apogee), the tides are not as spectacular.

Tides most commonly occur twice a day (diurnal). Tides can also occur as two high waters and two low waters each day (semi-diurnal). However, these periods do not happen at the same time each day. This is because the Moon takes slightly longer than 24 hours to line up again exactly with the same point on the Earth - about 50 minutes more. Therefore, the timing of high tides is staggered throughout the course of a month, with each tide commencing approximately 24 hours and 50 minutes later than the one before it. 

There are many factors involved in predicting the tides. In addition to the motion of the Moon and Sun described above, timing of the tides are also affected by the Moon's declination (angular height above the equator), local geography of the coastline, topography of the ocean floor, and depth of the water, among other considerations. Thus, the tides can't be perfectly predicted solely by astronomical calculations that track the Sun and Moon. For greatest accuracy, tide prediction tables always integrate data from actual observation, often over a period of many years.

The Different Kinds of Lunar Eclipse

A lunar eclipse can take three different forms:

  • A penumbral eclipse is when the moon passes through the Earth's penumbral shadow, but this is very difficult to see.

  • A partial lunar eclipse occurs when a portion of the moon passes through the Earth's umbral shadow. This is easy to see and can be viewed by the naked eye without any danger of damage to the eye whatsoever.

  • A total lunar eclipse occurs when the whole moon disappears as it passes through the Earth's umbral shadow. This is when the moon emits very different vibrant colors which are breathtakingly beautiful.

Watching Beauty In Action

Seeing a lunar eclipse depends on your location. When it is very visible in one part of the world, it can't be see in other parts. Only where it is night can you see the wonderful display of the eclipse. Such a phenomenon is very safe to watch. It is not even necessary to use a telescope and when you watch what a treat you are in for! The colors range from deep browns, very dark grays, various shades of brilliant reds and many shades of bright oranges. As the moon passes, these colors vary depending on the amount of dust in the Earth's atmosphere. If you are lucky enough to witness this spectacular show of beauty, the chances of seeing the same thing twice is pretty remote.

A lunar eclipse is a rare event because it doesn't happen on a monthly basis. It only happens when the moon is in direct alignment with the Earth and the Sun. The sun and the moon are actually on opposite sides of the Earth. Most of the time the moon is tilted in respect to the Earth's orbit and passes either above or below the line where all three celestial bodies are in a straight line. It is only about every six months that conditions may be right for a lunar eclipse.

Don't Miss the Next Show

Depending on where you live on this Earth, look up the time of the next eclipse in your area. Usually it is announced on the radio or television giving people ample time to enjoy the thrill of watching the Earth block out the moon with the most brilliant display. It is an amazing sight to see. For a closer look, use a telescope to bring the eclipse a little more up close and personal.

A Solar Eclipse

A solar eclipse is arguably one of the most thrilling sights on Earth. It occurs when the moon moves in front of the Sun. A solar eclipse happens slowly as the Moon starts its trip across the Sun. It begins like it is taking a small bite out of the Sun. As the Moon gets closer to the center of the Sun the darkness occurs more quickly. While a small crescent of the sun stays in the sky you can observe a unusual phenomenon with a solar eclipse - thin wavy lines appear that are light and dark that can be seen on light colored surfaces. These are called shadow bands and are caused by the distortion of Earth's atmosphere.

The Last String of Light Before Darkness

When the Moon has nearly blocked out the sun at the last few minutes before the total solar eclipse, you will see some points of light surrounding the edges of the dark Moon. These look like a string of beads wrapped around the edges and are appropriately called Baily's Beads. They are named after the astronomer Francis Baily who first noticed them in the 18th century. These beads only appear for a few brief minutes before the total solar eclipse occurs.

Beautiful Darkness Descends

When the Moon finally obscures the Sun completely, darkness occurs on Earth during the day. It is not quite like the darkness of night but more of a surreal, almost soft darkness. The sky near the horizon still appears bright and this produces a reddish glow and some very unusual shadows. When the total solar eclipse finally arrives, it gives an unearthly appearance. There is a pinkish glow, which comes from the edges of the moon before the total eclipse and sometimes a red cloud appearance arch above it. The pink glow is called Chromosphere and the red cloud is called solar prominence.

More to See Than the Eclipse

Of course, the solar eclipse is the ultimate reason that you look at the sky. However, there are smaller shows that are taking place at the same time. When the Sun is completely blocked, some of the brighter stars and planets become more visible. Sometimes you can see a small comet as it travels its path near the sun. Become aware of your surroundings here on Earth: birds don't chirp, bees stop flying and there is a stillness of the Earth that is strangely quiet. The temperature drops without the heat of the sun during a solar eclipse.

Note: To observe a solar eclipse safely, you need to wear protective eyewear because you should never look directly at the sun. It could result in damage to your eyes.

Lunar Eclipse Compared To Solar Eclipse

A "lunar eclipse" and a "solar eclipse" refer to events involving three celestial bodies: the Sun ("solar"), the moon ("lunar"), and the Earth. A lunar eclipse occurs when the Earth passes between the Moon and the Sun, and the Earth's shadow obscures the moon or a portion of it. A solar eclipse occurs when the Moon passes between the Earth and the Sun, blocking all or a portion of the Sun. 

An eclipse can be total, partial, or annular. A total solar eclipse is when the moon blocks out the Sun entirely, a partial eclipse is when it blocks out a portion of the Sun, and an annular eclipse is when the moon is at its furthest point in orbit. It will not cover the Sun completely that's when you can see a thin ring of light emerging from the outside rim of the moon. 

How are a lunar eclipse and solar eclipse different? 

A lunar eclipse occurs at night and a solar eclipse occurs during the day. There are only certain times when either of them can occur. A lunar eclipse can only occur when the moon is directly opposite the Sun in the sky — a full moon. Even though there is a full moon each month, obviously a lunar eclipse does not occur on a monthly basis because the Sun isn't exactly in line with the Earth and the moon. The moon's orbit is actually tilted 5 degrees more than that of the Earth; otherwise, we would see a lunar eclipse each month. 

We can see lunar eclipses more readily than solar eclipses, and it has to do with proximity. The Moon is much closer to the Earth (well over 300 times closer than the Sun!), so the Earth has a much greater chance of blocking sunlight to the Moon, compared to the Moon blocking light from the Sun. Also, a lunar eclipse can be seen from a greater portion of the Earth. Solar eclipses, on the other hand, are more rare and when they do happen can only be seen by a very narrow segment of people on Earth, for a short period of time. 

It is quite safe to watch a lunar eclipse with the naked eye, while watching a solar eclipse without eyewear protection can seriously damage your eyesight. You can use a telescope to get a clearer view of the moon during an eclipse and really see what is happening. 

A solar eclipse has always had a more profound effect on humans than a lunar eclipse. This is probably because of the importance of the Sun to all life on Earth. In ancient China, a solar eclipse was thought to be the dragon coming to eat the Sun. The effect that an eclipse has on all life on Earth is of particular interest to scientists. They eagerly await a solar eclipse because it helps them to gather more knowledge about the Sun and its position with respect to Earth.

Understanding The Moon Phases

Have you ever wondered what causes the moon phases? We all know that its appearance changes over time. But why? The good way to understand the phases of the moon is to examine an earth-moon-sun diagram: 

moon phases diagram

Diagram Explanation

The illustration may look a little complex at first, but it's easy to explain.

Sunlight is shown coming in from the right. The earth, of course, is at the center of the diagram. The moon is shown at 8 key stages during its revolution around the earth. The moon phase name is shown alongside the image. The dotted line from the earth to the moon represents your line of sight when looking at the moon. The large moon image shows what you would see at that point in the cycle. For the waning gibbous, third quarter, and waning crescent phases you have to mentally turn yourself upside down when imagining the line of sight. When you do this, you'll "see" that the illuminated portion is on your left, just as you see in the large image.

One important thing to notice is that exactly one half of the moon is always illuminated by the sun. Of course that is perfectly logical, but you need to visualize it in order to understand the phases. At certain times we see both the sunlit portion and the shadowed portion -- and that creates the various moon phase shapes we are all familiar with. Also note that the shadowed part of the moon is invisible to the naked eye; in the diagram above, it is only shown for clarification purposes.

So the basic explanation is that the lunar phases are created by changing angles (relative positions) of the earth, the moon and the sun, as the moon orbits the earth.

Moon Phases Simplified

It's probably easiest to understand the moon cycle in this order: new moon and full moon, first quarter and third quarter, and the phases in between.

As shown in the above diagram, the new moon occurs when the moon is positioned between the earth and sun. The three objects are in approximate alignment (why "approximate" is explained below). The entire illuminated portion of the moon is on the back side of the moon, the half that we cannot see.

At a full moon, the earth, moon, and sun are in approximate alignment, just as the new moon, but the moon is on the opposite side of the earth, so the entire sunlit part of the moon is facing us. The shadowed portion is entirely hidden from view.

The first quarter and third quarter moons (both often called a "half moon"), happen when the moon is at a 90 degree angle with respect to the earth and sun. So we are seeing exactly half of the moon illuminated and half in shadow.

Once you understand those four key moon phases, the phases between should be fairly easy to visualize, as the illuminated portion gradually transitions between them.

An easy way to remember and understand those "between" lunar phase names is by breaking out and defining 4 words: crescent, gibbous, waxing, and waning. The word crescent refers to the phases where the moon is less than half illuminated. The word gibbous refers to phases where the moon is more than half illuminated. Waxing essentially means "growing" or expanding in illumination, and waning means "shrinking" or decreasing in illumination.

Thus you can simply combine the two words to create the phase name, as follows:

After the new moon, the sunlit portion is increasing, but less than half, so it is waxing crescent. After the first quarter, the sunlit portion is still increasing, but now it is more than half, so it is waxing gibbous. After the full moon (maximum illumination), the light continually decreases. So the waning gibbous phase occurs next. Following the third quarter is the waning crescent, which wanes until the light is completely gone -- a new moon.

The Moon's Orbit

You may have personally observed that the moon goes through a complete moon phases cycle in about one month. That's true, but it's not exactly one month. The synodic period or lunation is exactly 29.5305882 days. It's the time required for the moon to move to the same position (same phase) as seen by an observer on earth. If you were to view the moon cycling the earth from outside our solar system (the viewpoint of the stars), the time required is 27.3217 days, roughly two days less. This figure is called the sidereal period or orbital period. Why is the synodic period different from the sidereal period? The short answer is because on earth, we are viewing the moon from a moving platform: during the moon cycle, the earth has moved approximately one month along its year-long orbit around the sun, altering our angle of view with respect to the moon, and thus altering the phase. The earth's orbital direction is such that it lengthens the period for earthbound observers.

Although the synodic and sidereal periods are exact numbers, the moon phase can't be precisely calculated by simple division of days because the moon's motion (orbital speed and position) is affected and perturbed by various forces of different strengths. Hence, complex equations are used to determine the exact position and phase of the moon at any given point in time.

Also, looking at the diagram (and imagining it to scale), you may have wondered why, at a new moon, the moon doesn't block the sun, and at a full moon, why the earth doesn't block sunlight from reaching the moon. The reason is because the moon's orbit about the earth is about 5 degrees off from the earth-sun orbital plane.

However, at special times during the year, the earth, moon, and sun do in fact "line up". When the moon blocks the sun or a part of it, it's called a solar eclipse, and it can only happen during the new moon phase. When the earth casts a shadow on the moon, it's called a lunar eclipse, and can only happen during the full moon phase. Roughly 4 to 7 eclipses happen in any given year, but most of them minor or "partial" eclipses. Major lunar or solar eclipses are relatively uncommon.

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