Today the
Earth is at its perhelion. In a couple of hours, I will be as close to the sun as I will be for the next
year.
How much closer?
About 5 million kilometers (or 3 million miles) closer than at the aphelion.
Still feels like winter in these parts.
Yep, but according to my almanac, today's the day.
So why is it cold outside?
Well, despite being closer to the sun, the northern hemisphere is tilted away from the sun. Around the aphelion, this neck of the woods will be tilted towards the sun, allowing more direct sun exposure.
Averaged over the whole planet, however, we'll receive about 6-7% more sunlight today than on July 5th.
So perihelion has nothing to do with the seasons?
The short answer is no--but if you hang around for a few tens of thousands of years, maybe.
The perihelion regresses over time--the date of the perihelion cycles over a period of about 21,000 years. In about 10,000 years or so, the perihelion will occur in July.
The ellipticity of the Earth's orbit varies in a 100,000 year cycle. If the orbit happens to be particularly elliptical at the same time the perihelion falls in the summertime in the northern hemisphere, well, maybe that would have an effect on iced tea sales.
More important is the Earth's tilt, which cycles through a 40,000 period--the more the tilt, the more severe the seasons.
A Serbian physicist Milutin Milankovitch devoted his professional life to developing a theory of longterm climatic changes based on the Earth's varying orbits and tilt, now known as the Milankovitch Theory. Climatologists have tried to correlate known ice ages with Milankovitch's data, with limited success.
With the onset of global warming attributed to CO2, we may cook ourselves long before the perihelion again falls in July.