happy perihelion

The solstices are the easiest sun-Earth relationships to identify. Humans have noted them for ages. But there's another relationship that depends on how distant (or near) we are to the sun. That distance changes over time. If Earth's orbit was perfectly circular, then we would always be the same distance from the sun, receiving the same amount of radiation from it. It's close but not exactly circular, so we orbit sometimes closer to the sun and sometimes farther away. Today (at 10:17am CST in Minneapolis), we reach our closest approach to the sun.

That circular shape is not permanent. It slowly changes over time in what are known as the Milankovich cycles, where the orbital eccentricity changes over geologic timescales. That variation (and others) is relevant to climate change.

When Earth’s orbit is at its most elliptic, about 23 percent more incoming solar radiation reaches Earth at our planet’s closest approach to the Sun each year than does at its farthest departure from the Sun. Currently, Earth’s eccentricity is near its least elliptic (most circular) and is very slowly decreasing, in a cycle that spans about 100,000 years.
- climate.nasa.gov

It also matters where Earth's land masses have migrated over these time spans. Currently, we're lucky that most of the land surface is in the northern hemisphere, pointed away from the sun during perihelion. Otherwise global heating would be worse than it is. Instead, Australia and South America (and the southern tip of Africa) bear the brunt of the heating, while southern oceans tend to reflect more of that radiant energy back into space. If the continents were in different positions, climate change would be even more problematic.

It's snowing in Minneapolis today, so I won't be looking sunward to note the holiday.