The retrograde motion refers to the backward motion of a planet. This is caused by the combined motion of both the planets and that of the earth's. When both planets move eastward around the Sun and then the planet which has a smaller orbit will tend to move faster. Basically, when a faster planet dominates the Earth when it comes to their rotations, this planet's motion against the stars would be seen from the Earth in reverse.

Every once in a while, however, a particular planet would do something relatively odd and this would be slowing down its motion to the point that it seems to have completely stopped for that period of time. Afterwards, it would begin moving towards the opposite direction, towards the west. For a second time, it would stop again and resume its normal movements. Ptolemy had this idea that all the planets were carried on a smaller circle and whose center moves along another larger circle around the earth which he referred to as the deferent.

Retrograde Motion

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If a body is rigid or the complete opposite – flimsy, tidal forces can tear the body in half. However, the Earth is an elastic sphere and the tidal force from the moon, in a way, distorts its shape although the mass or volume is not affected at all. Bulges' indicate this change in shape — an indication of the changes in the ocean levels due to tides, specifically.

The moon pulls on the planet as a whole, although its effect on solid Earth is only minimal – a few centimeters. It has more effect on the atmosphere and the hydrosphere. The oceans get attracted to the moon, causing high tides. However, it is noticeable that the nearer oceans are pulled more than the father oceans located at the side facing away wherever the moon is.

Tidal force also has a more general usage in astronomy, referring to the phenomenon where a body is subject to the gravitational force of a second body, while being subject to the gravitational force of a third body simultaneously, as in the case of the moon being primarily influenced by the Earth's gravity (due to its proximity) while being affected by the sun's gravitational pull as well.

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As an object travels up and increases its elevation, the pull of gravity gets weaker. However, it needs an initial velocity to gain enough kinetic energy (KE). This KE (0.5 times mass and square of velocity), will sustain the flight until it is out of gravity?s influence. That velocity is about 25000 mph or 7 mps.

However, it is a common misconception that an object must have this speed instantly to leave earth. Escape velocities also vary depending on the current elevation of the object. Any object can leave the planet at lower speeds?but with sustained thrust. Space shuttles, for example, do not achieve the escape velocity during take-off. They continuously defy gravity by burning tons of fuel. When they achieve a certain altitude, the influence of gravity will be minimal.

Explaining Escape Velocity

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Weight is equal to the force applied on a person or any object by gravity at the Earth’s surface. When measuring your weight, you measure it by the standard unit of pounds or kilograms. The mass of a person is different from weight because mass is the amount of material inside an object or a body. If you take an airplane on the moon, its mass remains the same but its weight is different than when it was on Earth because of the absence of gravity on the moon. The different planets have different levels of gravity therefore the weight of an airplane on Earth will not be the same on Jupiter.

When you are in an elevator that is free falling, your weight is at 0 because there is no force exerted on your body. If you can only measure yourself with a weighing scale while you are falling off a building, you will also experience no weight at all. This is called weightlessness or the absence of weight. When you are out in space, you will also be weightless because of the lack of gravity pulling you down.

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Tides are the periodic fluctuations of large bodies of water. They are caused by the gravitational interaction between the Earth and the Moon in what is generally called tidal forces. Although tidal changes may occur as a result of other factors like the Moon's altitude and distance, the positions of the sun and the Earth and the depth of ocean floors and coastal topography.

The Moon exerts more than twice the sun's gravitation force on earth, making it the most important factor. Its gravitational pull causes the oceans to bulge out to its direction. Another bulge appears on the opposite side due to the Earth's gravity too and the spherical shape is retained although the pull o the opposite side is weaker. Most places experience two high tides and two low tides every day, most because of the interplay of Earth's rotation, the moon's gravity and the sun's gravitational force.

There are generally four stages in the rise and fall in the bodies of water. First, the sea level rises over a period of several hours, ultimately covering the inter-tidal zone in what is called the flood tide. Then, water rises to the maximum level in what is called high tide. After that, water levels fall over the same period and revealing the inter-tidal zone in what is called as ebb tide and finally, the water stops falling and reaches low tide phase.

Tides can be especially weak or strong, with differences in water levels reaching 20%. Spring tides, which have nothing to do with the season, occur when the moon, sun and the Earth are in a row and there is a combined gravitational force of the Moon and the Sun. They usually occur during new and full moons.

Proxigean Tides are unusually high tides generally occurring during the new moon where the moon is between Earth and the Sun and when it is especially close to the Earth. Neap tides are weaker tides, occurring when the moon and sun are perpendicular to each other.

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