Theories of Geography Part 1 – Motions of Earth, Seasons and Eclipse

Motions of Earth

There are four kinds of Earth’s motions as follows:

  1. Earth’s rotation on its axis
  2. Earth’s precession movement which is very much similar to a spinning top.
  3. Earth revolution around the Sun
  4. Earth along with the entire solar system moves around the center of the Milky way Galaxy.

Earth’s Rotation and Precession Movements

Earth rotates around its own axis from west to east. When seen from the North Star Polaris {Alpha Ursae Minoris}, Earth turns counter-clockwise. Rotation of Earth results in days and nights. Earth’s rotation is mostly the result of angular momentum left over during the formation process of Earth.

There are three distinct motions, the most noticeable being Earth’s rotation. Earth rotates once every 23 hours, 56 minutes, causing our cycles of day and night.

Earth also has precession (a wobble of the rotational axis) and nutation (a back-and-forth wiggle of Earth’s axis), caused primarily by the gravitational pull of the Moon as it orbits Earth.

Precession and nutation, over long periods of time, cause Earth’s north and south poles to point toward different stars.

Solar Day

A Solar day refers to one complete rotation of Earth on its own axis relative to Sun. However, since Earth also revolves around the sun, there are three kinds of days recognized by astronomers viz. Apparent  or True solar day; Mean solar day and sidereal day. These are discussed as follows:

Apparent {True} Solar Day

Apparent solar day is the interval between two successive returns of the Sun to the local meridian. A sundial can measure the apparent solar day with limited precision. The length of true solar day keeps changing throughout the year. There are two reasons for this.

Firstly, Earth’s orbit around sun is an ellipse. The second law of Kepler says that in elliptical orbits, the line joining the planet and sun sweeps out equal areas during equal intervals of time.  Thus, earth moves faster when it is nearest to Sun (perihelion) and moves slower when it is farthest from Sun (Aphelion).

Secondly, Earth currently has an axial tilt of about 23.5° and remains tilted in the same direction towards the stars throughout a year. This implies that when a hemisphere is pointing away from the Sun at one point in the orbit then half an orbit later (half a year later) this hemisphere will be pointing towards the Sun. This effect is the main cause of the seasons.

What is obliquity of the ecliptic?

Earth’s orbital plane is known as the ecliptic plane, and so the Earth’s axial tilt is called the obliquity of the ecliptic. Due to Earth’s tilt, Sun moves along a great circle (the ecliptic) that is tilted to Earth’s celestial equator. When the Sun crosses the equator at both equinoxes, the Sun is moving at an angle to the equator, so the projection of this tilted motion onto the equator is slower than its mean motion; when the Sun is farthest from the equator at both solstices, the Sun moves parallel to the equator, so the projection of this parallel motion onto the equator is faster than its mean motion. The result is that apparent solar days are shorter in March (26–27) and September (12–13) than they are in June (18–19) or December (20–21). The true solar day tends to be longer near perihelion taking about 10 seconds longer and is about 10 seconds shorter near aphelion. It is about 20 seconds longer near a solstice and shorter by 20 seconds near equinox.

Mean Solar Day

The average of the true or apparent solar day over an entire year is called the mean solar day. It has 86400 seconds. Albeit, the amount of daylight varies significantly, the length of a mean solar day does not change on a seasonal basis. However, the length of the Mean Solar Day increases by 1.4 milliseconds per century. The astronomers have calculated that Mean Solar Day was exactly 86,400 (24 hours × 60 minutes × 60 seconds) SI seconds in approximately 1820AD and now it is 86400.002 SI seconds. The reason behind this slowdown is the net effect of tidal acceleration and global glacial rebound.

Tidal Acceleration

Tidal acceleration refers to the effect of the tidal forces between an orbiting natural satellite and the primary planet that it orbits. We know that Moon’s mass is a considerable fraction of that of the Earth. The Ratio of masses of moon and Earth is about 1:81. So these two bodies can be regarded as a double planet system, rather than as a planet with a satellite. The large mass of moon is sufficient to raise tides in the matter of earth. The water of the oceans bulges out along both ends of the axis, passing through the centers of Moon as well as Earth. This tidal bulge is shown below.

The average tidal bulge shown in above figure closely follows the Moon in its orbit. However, since earth also rotates, the rotation drags this bulge ahead of the position directly under the Moon. The arrow shown in the earth shows the direction of this drag. Due to the simultaneously forces of moon’s gravitational force giving rise to the bulges in ocean water and substantial amount of mass in these bulges of water dragged by earth’s rotation, this bulge is deviated from the line through the centers of Earth and Moon. This gives rise to a Torque which is perpendicular to the earth moon line. This torque boosts moon in its orbit and decelerates earth’s rotation. The above phenomenon is responsible for the slowing Earth’s rotation. Due to the tidal acceleration, Earth’s mean solar day extends by 2.3 milliseconds every century. However, due to glacial rebound, this extension gets reduced by 0.6 seconds per century. So the net effect on mean solar day every century is 1.7 milliseconds.


Global Glacial Rebound

The average position of water is always nearer the equator. During glaciations water is taken from the oceans and deposited as ice over the higher latitudes closer to the poles. These poles are close to the polar axis or rotational axis of the Earth. The moment of inertia of Earth-water-ice system gets reduced which is very much similar to a rotating figure skater bringing her arms closer to her body, the earth should spin faster. This process leads to an increase in the rotation speed of the Earth and therefore to a decrease of the length of day.

Sidereal Day

The spinning of the earth on its polar axis is in fact takes 23 hours, 56 minutes and 4.09 seconds for rotation through the 360 degree. This is called sidereal day. During the time needed by the Earth to complete a rotation around its axis (a sidereal day), the Earth moves a short distance (approximately 1°) along its orbit around the sun. So, after a sidereal day, the Earth still needs to rotate a small additional angular distance before the sun reaches its highest point. A solar day is, therefore, nearly 4 minutes longer than a sidereal day.

Precession Movement of earth

The Precession movement of Earth is very slow and proceeds in the direction of the opposite of Earth’s Rotation. The one cycle completes in 28000 years.  The reason of precession movement is gravitational attraction of Moon as well as Sun.  The slightly irregular movement of earth’s axis due to precession is called Nutation.

Earth’s Revolution

The orbit of the Earth is the motion of the Earth around the Sun every 365.242199 mean solar days. The orbital speed of Earth around the Sun averages about 30 kilometre per second or 108,000 kilometers per hour.  This speed is equivalent to cover earth’s orbit in 7 minutes and distance from moon to Sun in 4 hours.

Occurring of Seasons

The path of the Earth around the Sun is elliptical and slightly irregular due to gravitational attraction of moon and other celestial bodies. A constant angle is maintained between the earth’s axis and its plane of elliptic, which is called angle of inclination. As we know that Earth’s rotation axis is tilted by 23.44° with respect to the elliptic, and is always pointed towards the celestial poles when the earth moves around the Sun.

The above phenomenon gives rise to 4 seasons.



The solstice refers to the events when the Sun’s apparent position in sky reaches its northernmost or southernmost extremes. Solstice happens twice a year, and twice a year happen the equinoxes. Altogether, the four are considered to start 4 seasons. At the time of northern solstice, sun is perceived to be directly overhead the 23.44° north known as Tropic of Cancer. Likewise, at the southern solstice the same thing happens for latitude 23.44° south, known as the Tropic of Capricorn. The sub-solar point will cross every latitude between these two extremes exactly twice per year. The point where sun is perceived to be directly overhead is called subsolar point.

The Northern solstice happens at 20-21 June and Southern solstice happens at 20-22 December. Generally, Northern solstice happens at 21 June and Southern solstice happens on 21 December. At Northern solstice, the places which are located at Arctic circle, posited at latitude 66.56° north will see the Sun just on the horizon during midnight. And all the places north of Arctic Circle will see Sun above horizon for 24 hours. This is called Midnight Sun or a Polar Day. At Northern solstice which are located at Antarctic circle, posited at latitude 66.56° south will see the Sun just on the horizon during midday. And all the places south of Antarctic Circle will NOT see sun at anytime of the day. This is called Polar Night.

At Southern solstice, Polar day occurs at Southern Pole and Polar Night occurs at Northern Pole.

Uttarayan & Dakshinayan

For 6 months of the year, the Sun appears to be moving north. This Northward migration of Sun appears to begin after December 22 and is completed on June 21, when the Sun is directly overhead 23.44° North. This is called Uttarayan in India.

After June 21, for the next 6 months, Sun appears to be moving South and this southward migration appears to get finished , when Sun is directly overhead the 23.44° South. In India we call this apparent migration Dakshinayan.


At equinox, Sun is at one of two opposite points where the celestial equator and ecliptic intersect. Sun can be observed to be vertically overhead the Equator. Equinox happens around March 20/21 and September 22/23 each year.

Longest Days & Nights

When Sun is direct overhead on 23.44° north, it is called Longest Day in Northern hemisphere. So Northern Solstice represents the longest day of the Northern hemisphere and smallest night of the Southern Hemisphere. When Sun is direct overhead on 23.44° south, it is called Longest Day in Southern hemisphere. So Southern Solstice represents the longest day of the Southern hemisphere and smallest night of the Northern Hemisphere.

Perihelion and Aphelion

Earth travels 939,886,400 kms along its elliptical orbit in a single revolution. The average distance is 150 million kms, but the orbit is elliptical and there is the difference if 2.5 million kms.  Perihelion is the point when Earth is closest to Sun and it occurs around 3rd January. The distance is 147.5 million kms.

Aphelion is the point when Earth is farthest from the Sun and it occurs on July 4. The distance is 152.5 million Kms,

Perihelion is the point when Earth is closest to sun and it occurs on January 3. The distance is around 147.5 million Kilometers.

Speed of Earth is fastest at Perihelion and slowest at Aphelion (Kepler’s Second Law).  The following Graphic shows the Solstice, Equinoxes and Helions altogether:


An eclipse is the partial or total blocking of the light of one object by another.  In the solar system, relative positions of the Sun, Moon, and Earth create solar eclipses and lunar eclipses.

Frequency of Eclipses

Perfect alignments of the Sun, Moon, and Earth are relatively uncommon, because the plane of Earth’s orbit around the Sun (ecliptic plane) is not the same as the plane of the Moon’s orbit around Earth.  Thus, during the new moon or full moon phases when an eclipse might be possible, the Moon is usually located just above or below the straight line that runs between Earth and the Sun, so no eclipse occurs.  All three bodies viz. Earth, Moon, and Sun line up just right about twice a year.

Lunar Eclipse

A lunar eclipse occurs when Earth passes between the Sun and the Moon in such a way that the Moon moves into Earth’s shadow. When a partial lunar eclipse is going on, the curved shadow of our planet is apparent on the Moon’s face; the Moon looks kind of like it is in a crescent phase, but the terminator line (the line between light and dark) is not curved the same way. When a total lunar eclipse is happening, the entire Moon is in Earth’s shadow, and the Moon looks full, but glows only faintly red.

The reason is as follows: Earth’s atmosphere is dense enough to act a little bit like a lens, so it refracts a small amount of sunlight shining through it toward the Moon. This small fraction of light, which is mostly red because that is the color of light that refracts best, bounces off the Moon’s surface and comes back to Earth. Before and after totality, the direct sunlight reflected off the Moon is so strong by comparison that it drowns out this refracted light, so we normally cannot see it with our unaided eyes. During totality, however, the Earth-atmosphere-refracted light is quite visible as a soft reddish glow.


Solar Eclipse

A solar eclipse happens when the Moon is directly in line between Earth and the Sun. The Moon’s shadow sweeps across Earth’s surface; at those places where the shadow lands, an eclipse is seen. Like Earth’s shadow, the Moon’s shadow consists of two parts: a dark, central region called the umbra, and a lighter region called the penumbra that surrounds the umbra. Under the penumbra, a partial solar eclipse occurs. Under the umbra, a total eclipse or an annular eclipse is seen. Since the Moon travels in a slightly elliptical orbit around Earth, rather than in a perfectly circular path, its distance from Earth is not always the same.  If the Moon’s umbra falls on Earth’s surface when the two bodies are at a closer point in the Moon’s orbit, there is total solar eclipse. But if the Moon happens to be too far away from Earth at that time, the Moon does not cover enough of the sky to block the Sun’s rays entirely. In that case, the Sun is seen as a ring, or annulus, of light glowing around the silhouette of the Moon. During totality of a solar eclipse, the Sun looks like a perfectly black disk surrounded by glowing light. This light is actually the Sun’s corona, which is invisible under normal circumstances because the Sun is so bright. Away from the corona, the sky is dark, so planets and stars that ordinarily could be seen only at night become visible.

Frequency of Solar Eclipse at a particular location on earth

The entire process of a solar eclipse, from the beginning of partial coverage until the end, usually takes about an hour. However, the totality of solar eclipse lasts at most only a few minutes. Most total solar eclipses last between 100 and 200 seconds— just about two to three minutes. Furthermore, total solar eclipses can be observed only from narrow bands on Earth’s surface, and these bands change with each eclipse. In any given location on Earth, therefore, a total solar eclipse may appear only once every few centuries.

Why Moon blocks Sun so perfectly during solar eclipse?

The Moon’s diameter is just under 400 times smaller than the diameter of the Sun. Coincidentally, the Moon’s distance from Earth is also just under 400 times smaller than the Sun’s distance from Earth. That is why the Moon covers almost exactly the same amount of sky, when viewed from Earth’s surface, as the Sun. We are able to see only Corona during Total solar eclipse.


January 11, 2018

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