Perihelion

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

Perihelion is the point in the Earth's orbit where the Earth comes nearest the Sun. This happens around 3 January, at a distance of about 147 million km. It matters because it shows that distance from the Sun does not cause seasons. The Northern Hemisphere has winter at perihelion, even though the Earth is nearest the Sun then.

Explanation

Why the Earth has a nearest point

• The orbit is an ellipse, not a circle:
• The Earth goes round the Sun on an ellipse (a slightly stretched circle).
• So the Earth-Sun distance changes through the year.
• The nearest point is perihelion, about 147 million km, around 3 January.

• The farthest point is aphelion, about 152 million km, around 4 July.

• The solar constant (the energy falling straight on one square metre at the top of the atmosphere) is measured at the Earth's average distance from the Sun. At perihelion the Earth is closer than average, so it gets a little more than this.

How much more insolation at perihelion

• Insolation (incoming solar radiation) is the energy from the Sun that reaches the Earth.
• Energy falls with the square of the distance:
• This rule is called the inverse-square law.
• (152 ÷ 147)² ≈ 1.07.

• So the energy received changes by about 7% between perihelion and aphelion.

• This is much bigger than the change from sunspots:

• 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 through the year, Earth-Sun distance changes insolation far more than the sunspot cycle does.

Why it barely changes weather

• On 3 January the Earth gets slightly more insolation than on 4 July.
• Two things hide this small extra amount:
• the distribution of land and sea (land and water heat up and cool down at different speeds)

• the circulation of the atmosphere (winds that move heat around the globe)

• So perihelion has almost no effect on daily weather.

• Seasons come from tilt, not distance:
• The Earth's axis is tilted at about 23.5°.
• Tilt changes the angle of the Sun's rays and the length of the day.
• These two things, not distance, decide summer and winter.

Effect on tides

• Spring tides (extra-high high tides and extra-low low tides) come at new moon and full moon. At these times the Sun, Moon and Earth are in a line.
• When perihelion falls at a new or full moon:
• The Sun is nearest, so its pull on the oceans is a little stronger.
• This adds to the spring tide.

• The result is unusually large tides: very high high-tides and very low low-tides.

• Perihelion alone does not make every tide large. It does so only when it comes at the same time as a new or full moon.

Where it occurs

• The whole Earth, once a year: perihelion is a point on the Earth's orbit, reached around 3 January.
• Northern Hemisphere, including India: perihelion comes in winter. India is in its cool winter season even though the Earth is nearest the Sun.
• Southern Hemisphere: perihelion comes in summer. This is because the Southern Hemisphere is tilted towards the Sun in January, not because the Earth is nearer.
• The world's coasts: when perihelion falls at a new or full moon, coasts everywhere can see unusually large spring tides.

Don't confuse with

• Aphelion: the farthest point of the orbit (about 152 million km, around 4 July). Perihelion is the nearest point (about 147 million km, around 3 January).
• Solstice: a solstice depends on the tilt of the axis. It is the day the Sun is overhead at the Tropic of Cancer or Capricorn. Perihelion depends on distance. The two fall close together in the calendar, but they are different events.
• Spring tide: spring tides happen at every new and full moon, all year round. Perihelion only makes a spring tide larger when they come at the same time.
• Solar constant: this is measured at the Earth's average distance from the Sun. It is not the value at perihelion, which is slightly higher.

Prelims Hooks

• Perihelion ≈ 3 January, ≈ 147 million km; aphelion ≈ 4 July, ≈ 152 million km.
• Trap: "The Earth is nearest the Sun during the Northern Hemisphere summer" is wrong. The Northern Hemisphere has winter at perihelion.
• Insolation changes by about 7% between perihelion and aphelion. Over the 11-year sunspot cycle it changes by only about 0.1% [2].
• Seasons are caused by axial tilt (about 23.5°), not by Earth-Sun distance.
• Perihelion + new or full moon → unusually large spring tides.
• The extra insolation at perihelion is hidden by land-sea distribution and atmospheric circulation. So it has almost no effect on daily weather.

Mains Points

• Distance vs tilt (GS-I):
• Insolation changes by about 7% between perihelion and aphelion.
• Yet the Northern Hemisphere has winter at perihelion, and land-sea distribution and winds hide the distance effect.

• Use this to show that the angle of the Sun's rays and the length of the day, both set by the tilt, control the seasons and how insolation spreads across latitudes.

• Natural changes in solar energy (GS-I / GS-III):

• Changes in distance through the year (about 7%) are much larger than changes over the sunspot cycle (about 0.1%) [2].

• Both changes are natural. Knowing how small they are helps separate them from warming caused by greenhouse gases.

• Tides and coasts (GS-I):

• When perihelion falls at a new or full moon, the Sun's stronger pull gives unusually large spring tides.
• Very high high-tides can matter for flooding in low coastal areas. Very low low-tides can matter for ports and navigation.

Related concepts

• Insolation
• Solar constant
• Shortwave and longwave radiation
• Planck's law
• Aphelion

Read more

• Solar radiation and insolation