My Dark Sky

Today, 4 July, the Earth reached its furthest point from the Sun for this year. We call this point in space aphelion.

Usually we say that the Earth is 1 astronomical unit (AU) from the Sun, which is equal to roughly 150 million km. This is just the average value because the orbit of Earth around the Sun is not a perfect circle, or else we will always be at the same distance from the Sun.

Our orbit is elliptical, not a lot, but slightly with an eccentricity* of 0.017. Hence, there will be time when our planet is nearer to our star, and time when it is further away. January is the time when it’s nearest (called perihelion), and 6 months later, will be the time when it is farthest away (called aphelion).

The Earth is 147 million km from the Sun at perihelion and 152 million km at aphelion, so today our Earth is 5 million km further away than it is in January, or 2 million km further away than the average distance.

It was Kepler who first explains that planets are orbiting the Sun in a path described as an ellipse. According to Kepler’s 2nd Law, at our furthest point from the Sun, the Earth is travelling most slowly in its annual orbit around the Sun.

And also since we are furthest away, the Sun’s angular size is the smallest during the year, if you ever notice it (I don’t think a casual observer ever notice the change in size of the Sun). A good way to compare the size of the Sun is by taking a picture on January and another one is July, and then put them side by side.

WARNING: DO NOT LOOK AT THE SUN DIRECTLY WITHOUT ANY SAFETY FILTERS OR EQUIPMENT!!!

*The value is between 0 and 1. Eccentricity of zero means a perfect circle, the higher the value means the more it deviates from a circle and its shape becomes long and skinny. It is equal to one in the case of a straight line.

~ by thChieh on July 4, 2008.

Posted in General Astro Talk