Perihelion Day

Sun closest to the Earth

Today, January 4 is Perihelion Day. From the Greek, “peri” means “close” (think periscope or perimeter) This is the day of the year when the sun is closest to the Earth. Today at 8:28 am, the center of earth is 91,405,993 million miles from the center of the sun.

The opposite of Perihelion Day is Aphelion Day. (“apo” means “far”. That’s when the sun is farthest away from the earth. This year that occurs on July 5. T the sun will be 94,502,939 miles away from the Earth. Viewed from Earth, that means the sun looks ever-so slightly bigger in January (about 7% bigger) and ever-so-slightly smaller in July.

So, if the sun is 3 million miles closer to the earth today than it is during the first week of July, why is it hot in July and cold now? Shouldn’t it be just the opposite?

There are two big reasons for the difference. A much bigger factor that the closeness of the sun is the low angle of the sun’s rays in Northern Hemisphere winter. At solar noon today, the sun is 24 degrees above the southern horizon. At the summer solstice in June, the sun is 70 degrees above the southern horizon.

If the sun was farthest away in January and closest in July, then it would be 3 degrees colder in January and 3 degrees hotter in July. I don’t know about you, but I’d just as soon keep things the way they are.

This graphic is from the Little Rock, Arkansas National Weather Service. It’s for 2022, so the 91,406,842 number in the paragraph above on the graphic is ever-so-slightly different from the 91,405,993 figure we have here in 2025.

Every 58 years, the date of Perihelion moves forward one day. This is part of a set of cyclical patterns known as the Milankovitch cycles. Back in the year 1246, Perihelion Day was on the same day as the Winter Solstice (a few days before Christmas). By the year 6430, perihelion will happen simultaneously with the March Equinox.

Here’s something else that’s interesting. As the earth moves around the sun, it travels at approximately 67,000 mph. However, when the sun is closer to the Earth in the Northern Hemisphere in winter, the Earth “speeds up” just a bit. That means the seasons are not quite the same length. If you think about it…February has only 28 days (29 in a leap year). July and August both have 31 days. In the Northern Hemisphere, spring and summer are a few days longer than fall and winter. In 2022, winter was (Winter Solstice to Spring Equinox) 89 days, 23 hours and 34 minutes. Spring was 92 days, 17 hours and 40 minutes long. Summer was 93 days, 15 hours and 49 minutes long. In the Southern Hemisphere, spring and summer are slightly shorter than fall and winter.

Again, here in Michigan, we calculate that because Perihelion Day occurs in January – in the middle of winter – that we are roughly three degrees warmer in January and three degrees cooler in July, when the sun is farther away. This (slightly) minimizes the difference between winter and summer. By my calculation, in the year 12,368, perihelion will be on July 4. When that happens…it’ll be about 3 degrees hotter in July and 3 degrees colder in January (in Michigan). I’m personally not looking forward to that.

We are also now at our latest sunrise of the year at 8:13 am. Sunsets are already starting to get later in the evening, albeit at a relatively slow pace in January. The sunset today (1/4) is at 5:21 pm. That’s a gain of around 8 minutes since the earliest sunset back on Dec. 9. So, we’ve gained around 8 minutes of total daylight since the Winter Solstice back on Dec. 21.

Average temperature lags the position of the sun by about one month. So, temperature-wise, the mid-point of winter is around January 20. The second half of winter is brighter, because we get a few more sunny days and daylight is longer than in the first half of winter.

Sunrise today in Grand Rapids is at 8:13 am and sunset is at 5:21 pm. Tomorrow (Sun. 1/5), sunset will be at 5:22 pm. Daylight Sunday will be one minute and three seconds longer than today (Sat. 1/4).

Up in Alaska today, Fairbanks (a bit south of the Arctic Circle) has a sunrise at 10:48 am and a sunset at 3:03 pm. At solar noon, the sun is only 3 degrees above the horizon. So unless you have a clear view to the horizon, you probably won’t see the sun at all. However, in early January they are gaining around 4 1/2 minuts of daylight each day. By the end of the month it’s about 6 1/2 minutes of daylight each day.

At Utqiaġvik - the northern tip of Alaska - they are north of the Arctic Circle. They won’t see the sun until January 22. Then, just the top tip of the sun will appear above the horizon for 48 minutes,

This is a “daytime” pic. from New Year’s Eve at Utquigvik. Even though the sun stays below the horizon in winter, they do get a short period of dim twilight at midday.

Yesterday, Utqiagvik AK had a high temperture of -15F. Fairbanks had a high of -28F and a low of -38F. Tok, was the coldest spot in Alaska with a low temperature of -53F.

Thanks for reading my blog. Have a great weekend.