June 14–20

The Sun stands still

Summer officially arrives in the Northern Hemisphere this week, in the form of the summer solstice, the instant the Sun reaches its most northerly position for those living north of the equator. In the Southern Hemisphere, meanwhile, the Sun reaches its lowest point in their sky, hence this week marks the beginning of southern winter.

On the first day of summer, the Sun is directly overhead at noon for people living at latitude 23.3°N, a region denoted as the Tropic of Cancer. There's nothing cryptic about this value. It's just how much Earth's rotational axis is tilted with respect to the plane of the solar system. If Earth's axis were tilted more, say 30°, the Sun would be overhead at that latitude instead.

As we orbit the Sun, Earth's tilted axis makes the Sun appear to gradually move north to south and back again from any given location in the world. But only in regions within 23.3° of the equator does the Sun actually ever stand directly overhead. If you live at latitude 30°N, the Sun is still 6.7° south of your zenith on the first day of summer (30°—23.3°); from latitude 40°N, the Sun is 16.7° south of the zenith on that day. (For more on Sun angles, see March 15–21.)