Science & history · Light and energy
Sunlight is about 7% stronger in January than in July
The amount of sunlight arriving at the top of the atmosphere changes over the year, because Earth's distance from the Sun changes.
Sunlight at Earth's distance today
Worked out in your browser for today's date.
The arithmetic
Irradiance falls with the square of distance. USNO dates perihelion to 3 January 2026 and aphelion to 6 July (seasons data); at those moments this site's calculation puts Earth 0.983 and 1.017 astronomical units from the Sun (method). Squaring the ratio, (1.017 ÷ 0.983)², gives about 1.07: roughly 7% more sunlight arrives in early January than in early July.
Applied to the IAU's 1361 W/m² at one astronomical unit, that is about 1408 W/m² at perihelion and about 1317 W/m² at aphelion, above the atmosphere.
So why is January cold in the north?
Because the seasons come from the tilt of Earth's axis, not from distance. In January the Northern Hemisphere is tilted away from the Sun: shorter days and a lower Sun outweigh the extra 7%.
Sources
- US Naval Observatory Astronomical Applications API (opens in a new tab) for independent sunrise, sunset, civil twilight, transit and seasons for the engine's tests · checked 2 October 2026
- Cool Moon Facts How the Sun pages calculate for the engine, its definitions, its independent checks and its limits
- International Astronomical Union IAU 2015 Resolution B3: nominal solar and planetary conversion constants (Mamajek et al.) (opens in a new tab) for nominal radius 695,700 km, irradiance 1361 W/m², luminosity 3.828×10²⁶ W, effective temperature 5772 K · checked 2 October 2026
- NASA Space Place What Causes the Seasons? (opens in a new tab) for tilt, not distance; perihelion in January · checked 2 October 2026