# How can I find the distance to the Sun on any given day?

*What was the distance of the sun from earth on May 22, 2002? How can I find this out for any given date of the year?*

This isn't a particularly hard calculation, but it will require some high school maths.

You need to know the equation for the ellipse of the Earth's orbit around the Sun, and the date at which the Earth is at perihelion (meaning the closest point to the Sun), and then you're set.

Perihelion is listed for 1992-2020 on the US Naval Observatory site. The orbit of the Earth around the Sun is an ellipse with semi-major axis of 149.6 million kilometres and an eccentricity of 0.017. Below is a diagram of an ellipse.

You can figure out the distance of the Earth from the Sun using the equation

r = a(1-e*e)/(1+e cos(theta))

where r is the distance from the Sun, a is the semi-major axis, e is the eccentricity and theta is the angle around from perihelion. These things are shown on the diagram above. The position of the Sun is at one focus of the ellipse, labelled 'F'. The symbol for the Greek letter theta is used (the O with a line through it).

We know a and e. The tricky thing here is linking theta with the time (and therefore the date). A good approximation is to assume that the Earth orbits at a constant angular rate, meaning that the time in days since perihelion is

time = theta*365.25/360.

(because there are 365.25 days in the year and it takes 360 degrees to go all the way round once).

This isn't quite true, because, as Kepler figured out in the 1600s planets orbit a little bit faster when they are near the Sun than when they are away from it. That makes relating theta with the time since perihelion a little bit trickier, but it can be done, and you can read about it here or in books on celestial mechanics if you are so inclined (note that theta has the technical name of "true anomaly" and is labeled "nu" in the discussion linked).

I actually did this small calculation and created a graph of the result which is shown below. Note that the distance is incorrectly labelled as being in millions of kilometres on the graph, when actually it is in hundreds of millions of kilometres.

May 22 2002 is 140 days after the perihelion of Jan 2 2002, meaning that the Sun was about 151.5 million kilometres away from the Sun (as you can read off the graph). The mean distance to the Sun is 149.6 million kilometres, so on May 22 2002 the Earth was 1.3% further from the Sun than average. The maximum variation is about 1.5%, which (along with the graph) will tell you than May 22nd was pretty close to aphelion (the furthest point from the Sun).

You can use the graph above to find the distance to the Sun on any day of any year once you know how many days after the last perihelion that day was.

# Still Curious?

**Get More 'Curious?' with Our New PODCAST:**

- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.

**Related questions:**

- How big a change in the Earth's orbit would be required to destroy all life?
- Is the distance from the Earth to the Sun changing?
- How critical is the Earth-Sun distance in maintaining our average temperatures on Earth?
- How do you measure the distance between Earth and the Sun?
- How fast does the Earth go at perihelion and aphelion?

**More questions about The Earth:** Previous | Next

# How to ask a question:

If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

Main Page | About Us | For Teachers | Astronomy Links | Ask a Question | View a Random Question | Our Podcast

Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't existURL: http://curious.astro.cornell.edu/question.php?number=582

This page has been accessed *50970* times since October 20, 2003.

Last modified: *September 7, 2007 2:46:21 PM*

Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.

**Warning: Your browser is misbehaving! This page
might look ugly.** (Details)