Aphelion

Geography glossary

Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"; Class 11, Ch 13 "Movements of Ocean Water"

Meaning

Aphelion is the point in the Earth's orbit where the Earth is farthest from the Sun. It is about 152 million km away, and the Earth reaches it around 4 July each year. It matters because it shows that distance from the Sun does not make the seasons. At aphelion the Earth gets slightly less sunlight, yet the Northern Hemisphere is in summer.

Explanation

Why there is a farthest point

  • The Earth's orbit is an ellipse. An ellipse is a slightly stretched circle.
  • The Sun does not sit at the centre of this ellipse.

  • So the Earth's distance from the Sun changes through the year.

  • There are two extreme points:

  • Perihelion is the nearest point: about 147 million km, around 3 January.

  • Aphelion is the farthest point: about 152 million km, around 4 July.

  • The gap between the two is about 5 million km. This is small next to the total distance, so the orbit is very nearly a circle.

Effect on insolation

  • Insolation (incoming solar radiation, the Sun's energy that reaches the Earth) is slightly less at aphelion.
  • Why: the inverse-square law.
  • The energy received falls with the square of the distance.

  • (152 ÷ 147)² ≈ 1.07. So the Earth gets about 7% more energy at perihelion than at aphelion.

  • This is a much bigger change than the one the Sun itself makes.

  • Sunspots are dark, cooler patches on the Sun. Their number rises and falls over about 11 years.
  • Over one sunspot cycle, the solar constant changes by only about 0.1% [2].

  • So over a year, distance changes the energy the Earth receives far more than the sunspot cycle does.

  • Link to the solar constant:

  • The solar constant is measured when the Earth is at its average distance from the Sun.
  • It is about 1,361 W/sq m (SORCE) [2]. The NCERT value is 1.94 calories per sq cm per minute.
  • At aphelion, the energy reaching the top of the atmosphere is a little below this value.

Why aphelion barely affects weather or seasons

  • Land, sea and winds hide the small difference.
  • The distribution of land and sea and the circulation of the atmosphere (winds) cancel out the effect of distance.

  • So aphelion has almost no effect on daily weather.

  • Seasons come from the tilt of the axis, not from distance.

  • The Earth's axis is tilted by about 23.5°.
  • The tilt changes the angle of the Sun's rays and the length of the day.
  • In July, the Northern Hemisphere leans towards the Sun. It gets steeper rays and longer days, so it has summer, even though the Earth is at its farthest point.
  • If distance caused seasons, the whole Earth would have winter in July. It does not: the Southern Hemisphere has winter then and the Northern Hemisphere has summer.

Effect on tides

  • At aphelion the Sun's pull on the Earth is a little weaker.
  • So tidal ranges are smaller than average. Tidal range is the difference between high tide and low tide.
  • At perihelion the opposite happens. If it falls on the same days as a new moon or full moon (when spring tides happen), the tides are unusually large: very high high-tides and very low low-tides.

Where it occurs

  • The whole Earth: aphelion is a point in the Earth's orbit, so the whole planet is at aphelion at the same moment, around 4 July.
  • Northern Hemisphere (including India):
  • It is summer here at aphelion.

  • In India, early July is the southwest monsoon season. The country is warm even though the Earth is at its farthest from the Sun.

  • Southern Hemisphere: it is winter here at aphelion. The cause is again the tilt, not the distance.

  • Oceans and coasts everywhere: tidal ranges are smaller than average around aphelion.

Don't confuse with

  • Perihelion: the nearest point to the Sun (about 147 million km, about 3 January). Aphelion is the farthest point (about 152 million km, about 4 July).
  • Summer solstice (21 June): this depends on the tilt of the axis. On this day the Sun is overhead at the Tropic of Cancer. Aphelion depends on the shape of the orbit. The two fall close together, but they have different causes.
  • Apogee: the point where the Moon is farthest from the Earth. Aphelion is about the Earth and the Sun. Both give smaller tides.
  • Solar constant: this is measured at the Earth's average distance from the Sun. At aphelion the energy received is slightly less than the solar constant.

Prelims Hooks

  • Aphelion ≈ 4 July, ≈ 152 million km. Perihelion ≈ 3 January, ≈ 147 million km.
  • Trap: "The Earth is nearest to the Sun during the Northern Hemisphere summer" is wrong. In the northern summer the Earth is at aphelion, its farthest point.
  • Insolation differs by about 7% between perihelion and aphelion. The sunspot cycle changes it by only about 0.1% [2].
  • Seasons are caused by the tilt of the axis (about 23.5°), not by the Earth's distance from the Sun.
  • Aphelion → tidal ranges smaller than average. Perihelion + new or full moon → unusually large spring tides.
  • The energy received falls with the square of the distance (the inverse-square law). That is why a small change in distance causes about a 7% change in energy.

Mains Points

  • Distance vs tilt (GS-I):
  • The Earth is farthest from the Sun at aphelion, and insolation is about 7% lower than at perihelion. Yet the Northern Hemisphere has summer at this time.

  • This shows that the angle of the Sun's rays and the length of the day, both set by the axial tilt, control the seasons and how insolation is spread across latitudes.

  • Why the distance effect is hidden (GS-I):

  • The distribution of land and sea and the circulation of the atmosphere cancel out the small difference in insolation.

  • So aphelion has almost no effect on daily weather. This is a good example of how the Earth's surface and winds share out energy.

  • Tides and coasts (GS-I/GS-III):

  • Tidal ranges are smaller around aphelion and largest when perihelion falls on the same days as a spring tide.
  • Planning for coastal flooding, ports and tidal energy has to account for these yearly changes in tidal range.

Related concepts

Read more

Sources

  1. 1Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"; Class 11, Ch 13 "Movements of Ocean Water" (primary)
  2. 2NASA Goddard Space Flight Center: Solar Irradiance, Scienceearth.gsfc.nasa.gov · tier 2