Earth’s Revolution: Equinox, Solstice, Seasons, Perihelion and Aphelion

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.

Solstice

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.
  • In 2010, Northern solstice happened at 21 June and Southern solstice will happen 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 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. Dute to this, In India we call this Uttarayan.

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.

Equinox

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: On around January 3rd, Earth is closest to sun and distance is around 147.5 million Kilometers. This is called Perihelion.
  • Aphelion: On about July 4th earth is Farthest from Sun and this is called Aphelion.

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:

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