Observations

# Do you feel we are going too fast and need to slow down a little? You are right, the Earth is at moving at its fastest for the year, but it is about to start slowing down

The variation in the Earth’s distance from the Sun in astronomical units. An astronomical unit is the average distance of the Earth from the Sun and is approximately 150 million km. Diagram Nick Lomb

Today the Earth is at perihelion, that is, at its closest to the Sun in its slighly elliptical or oval-shaped path around the Sun. As you can see above, the variation is small with a plus or minus variation from the mean of only 1.7%. For other planets the variation can be much higher: for Mars it is 9.4% while for the dwarf planet Pluto the variation is as much as 24.4%.

The velocity of the Earth during 2011. for the calculations behind this diagram see this previous post. Diagram Nick Lomb

The Earth is moving fastest when it is closest to the Sun. This is Kepler’s second law and is purely a consequence of the law of conservation of angular momentum. You can see the law in action if you watch spinning ice skaters who slow down when they extend their arms and speed up when pulling their arms close to their bodies.

Today the Earth is moving around the Sun at the great speed of 109 025 km per hour. If you are careless enough to exceed the speed limit on a public road, you could try explaining to the police officer that your excess speed is trivial compared to the speed of the Earth. Then again, you may just get the officer angry and that would not be a good idea.

The diagram indicates that the velocity will remain high for the next few weeks and then start dropping until the Earth reaches aphelion, its greatest distance from the Sun for the year, on 5 July 2011.

If you think that in July we can all relax knowing that the Earth is moving at its slowest in its annual journey around the Sun, then think again. As well as moving around the Sun, the Earth accompanies the Sun on in its long journey around the centre of the Milky Way galaxy with a velocity about seven times that of the Earth moving around the Sun. Then the Milky Way itself is moving through space. Summing up these motions the Earth is moving through space at a velocity 12 times that of its velocity around the Sun. We are moving at a velocity of about 1.3 million km per hour towards the region of space denoted by the constellation of Leo the Lion.

Even if we are standing still, we are not where we were yesterday and not where we will be tomorrow!

## 2 responses to “Do you feel we are going too fast and need to slow down a little? You are right, the Earth is at moving at its fastest for the year, but it is about to start slowing down”

• Les Dalrymple says:

Whoops! Just after my post I noticed a lttle boo-boo in my maths regarding the number of days between the Equinoxes.

23 September 2010 to 20 March 2011 = 178 days (non-leap years)

20 March 2011 to 23 September 2011 = 187 days.

making the disparity in the number of days between the two equnoxes even more apparent. But, even if we went 23rd March to 23rd September (6 months exactly — or a half calander year) the difference is still 3 days between one half of the year and the other half and the root cause is the Earth’s elliptical orbit.

• Les Dalrymple says:

It isalso this differing pace around the Sun from one part of our orbit to another that produces the disparity in the number of days between the two equnoxes — even though the two equinoxes are a 6 months apart (give or take a few hours).

September Equinox to March Equinox = 181 days to cover the 1st half of our orbit in non-leap years (182 in leap-years).

March Equinox to September Equinox = 184 days to cover the 2nd half of the orbit.

Three-odd days difference due to the Earth’s speeding during the southern hemisphere summer and slowing during the southern hemisphere winter. Though I add that the seasons do not cause the speed-up and slow-down of the Sun, nor vice-versa.

But, it is also a fact that because we are somewhat closer to the Sun (the difference is about 3 million km) at Perihelion on 3 January than at Aphelion 4 July 2011, the Earth’s surface receives 7% more energy per m^2 on Perihelion day that we do at Aphelion day — but this doesn’t cause the seasons either!