It's Perihelion Day!

Today at 8:50 am ET (1350 UTC) the distance between the Earth and Sun was the smallest it will be this year (perihelion). On July 4, 2020, we were the furthest from the Sun (aphelion). Here is a picture comparing the Sun as seen by HMI on those two days.

The left half shows what the Sun looked last July. The right half is an image from today. The Sun appears a little larger today because of our elliptical orbit around the Sun. This difference also means the telescopes on SDO were designed to fit the Sun at perihelion into the images. I made this from the images on the SDO website. You can make one yourself using the other wavelengths available there.

For every minimum distance of an orbit there is also a maximum distance. For the Earth and Sun this is called aphelion and will next occur on July 5, 2021 6:27 pm ET (2200 UTC).

Happy Perihelion Day!

Happy Perihelion! Yesterday at 0636 UTC (1:36 am ET) the Earth was as close to the Sun as it will be in 2015. This is called perihelion. The distance change is small (about 1.67% closer at perihelion and 1.67% further away at aphelion in June), but we see the Sun as a little brighter today.

Our orbit around the Sun is almost a circle right now. That means our seasons are caused by tilt of our rotation axis. But there have been times in the past when our orbit was more of an oval. It's fun to think about what that would do to our seasons and climate. The Lensman books by E. E. Doc Smith included a nemesis with a home planet that had an extremely elliptical orbit and the bizarre evolutionary adaptations that led to.

Happy Perihelion!

Today, January 4, at 1200 UTC (7 am EST), the Earth was at perihelion. This is when we were closest to the Sun in our orbit around the Sun. At perihelion, the center of the Earth was a little over 147 million km (about 91.4 million miles) from the center of the Sun. Why do we care?

The Earth orbits the Sun along an ellipse. That means sometimes we’re farther from the Sun and sometimes closer. The average distance of the Earth from the Sun is about 150 million km (about 93 million miles, one Astronomical Unit or AU). Perihelion is the starting time for that orbit. The distance of perihelion is also important. The perihelion distance of the Earth is about 1.67% closer than the average distance.

When the Earth is closer to the Sun it receives more sunlight and when it is further away it receives less sunlight. That means the timing and distance of perihelion can affect our climate. That doesn’t mean that being closer to the Sun automatically means warmer weather.

Our seasons are caused by the tilt of the earth’s rotation axis compared to our orbit around the Sun. In January the northern hemisphere of the Earth is tilted away from the Sun and the northern latitudes have winter. At the same time those living in southern latitudes experience summer. Because the tilt determines the seasons we mark our seasons by the solstices and equinoxes.

But the changing distance does affect our climate. It appears to make southern winters a little less frigid and northern summers a little milder. In the past the difference was more dramatic.

The timing of perihelion in the year changes slowly. Right now perihelion happens in northern winter, while 11,000 years ago it happened in northern summer. That changed the severity of the seasons.

If we wait even longer (several hundred thousand years) the shape of the Earth’s orbit also changes, with the eccentricity changing from nearly 0 (almost circular) to the current value of 0.0167 to 0.06. A circular orbit means the distance to the Sun does not change, while a more elliptical orbit means the amount of sunlight hitting the Earth changes even more during the year. Right now the amount of sunlight hitting the Earth at perihelion is about 6.5% greater than what hits the Earth at the furthest distance. At an eccentricity of 0.06 that would increase to 27%.

These are just two of the Milankovitch cycles that were developed to explain glaciations.

SDO instruments were built to allow for the changing distance and apparent size of the Sun during a year. But it’s nice to know that those changes do other things as well.

Have a Happy Perihelion Day!