Earth passed the December solstice last month, but another milestone comes tomorrow (Jan. 3, 2011). It’s not one you would normally notice, nor one that you would logically expect. As the Earth orbits the sun, its distance varies by up to 3 million miles. Tomorrow, Earth is at perihelion, or its closest point to the sun of the year.
Now, if you live where the temperatures are frigid and there is snow on the ground (as there is here in Denver), you might reasonably question this fact. It seems logical that it would be warm when we are closer to the sun, and colder when we are farther away. But the truth is that this slight change in distance (about 3 percent) has little if any bearing on Earth’s seasonal temperatures. Rather, it is our tilt toward or away from the sun that causes the seasons. In summer we are tilted sunward and the sun is high in the sky so it gets hot. In winter the opposite is true. You can measure the height of the midday sun yourself to prove this. See my earlier blog, What’s noon to you?. Do this in several seasons and you will note a big difference.
Tomorrow – Monday, January 3 at 19 hours Universal Time (1 p.m. Central Time) – Earth is closest to the sun for all of 2011. This is the time that the Earth reaches perihelion, meaning closest to the sun. At that time, the sun will be about 147,096,000 km (about 91,402,000 miles), compared to about 152,104,000 (94,513,000 miles) in July.
You can also measure the actual size of the sun with a ruler, tape measure and small mirror. It does require a very small amount of math, but it is great for a school project. Just project the light of the sun from a small mirror onto a darkened wall.
Then measure the diameter of the projected image (d) and the distance between the wall in the mirror (l).
These are in the same ratio as the actual diameter of the sun (D) to the true distance to the sun (L).
So you can set up a simple equation and solve:
D = (d/l)*L
Be sure that your measurements for d and l are in the same units. Then if your distance to the sun (L) is in miles, your resulting value for the diameter of the sun (D) will be in miles as well.
This is a variation on the “pinhole” projection method and provides an actual image of the sun if the size of the mirror is very small compared to the distance of projection. A quarter-inch mirror at about 16 – 20 feet is good. The exact shape of the mirror is unimportant. If you are too close, or the mirror is too large, the projected image will not be a real image of the sun, so be careful with this. Also be careful not to look into the reflected beam or allow any child or animal to do so. It would like looking at the sun directly and can ruin your eyes.
I’ll keep this short, so I won’t add any more details here, but you can find much more in this activity I’ve used with students for years: The Diameter of the Sun.
By the way, you can do this any day of the year, not just the day of perihelion. To be most precise, you should use the exact distance to the sun on that day, but if you do not know it, it should be fine to use the average distance of about 93,000,000 miles (149,600,000 km).
Larry Sessions has written many favorite posts in EarthSky's Tonight area. He's a former planetarium director in Little Rock, Fort Worth and Denver and an adjunct faculty member at Metropolitan State University of Denver. He's a longtime member of NASA's Solar System Ambassadors program. His articles have appeared in numerous publications including Space.com, Sky & Telescope, Astronomy and Rolling Stone. His small book on world star lore, Constellations, was published by Running Press.