Science Questions

Is Earth always in the same location on your birthday?

Mon, 4th Jul 2016

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Pia Asa asked:

My daughter recently turned 10 and we started talking about planets rotating around the sun (as you do).


We were wondering whether you end up in exactly the same spot in the universe on your birthday ie since the earth goes around the sun in 365 days, does that mean that each year on your birthday you would be in exactly the same spot in the universe with respect to the sun? (all the other planets would be in different places wouldn't they?)


Or is it constantly shifting, and if so, then how?


Chris Smith put this to Andrew Norton, astronomer from the Open University...Birthday Candles

Andrew - Well, thatís a really good question. You're absolutely right. In one year, the Earth will return to the same spot with respect to the sun. but the sun is moving through space and the sun is moving, dragging all the rest of the planets in the Solar System with it. the Sun is moving with respect to the nearby stars. All those nearby star,s including the Sun, are in turn rotating around the galaxy Ė our galaxy, the Milky Way. Our galaxy, the Milky Way is also moving through space with respect to the other galaxies in our local group of galaxies. That local group of galaxies including the Andromeda galaxy, is moving through space with respect to the Virgo Supercluster of galaxies, of which weíre a part, and that Virgo Supercluster of galaxies is itself moving through space with respect to the cosmic microwave background, the background glow of the Big Bang, which we can imagine as a sort of static reference against which everything else moves. So, everything is moving.

Kat - Also, thatís assuming that itís exactly 365 days to go around the Sun, but itís not, is it?

Andrew - Itís not. Itís about 365 and a quarter days, which is why we get that extra day every leap year, every four years. So yeah, there's a little bit of adjustment due to that as well. But with everything else going on. Itís a lot of movement.

Kat - Who cares a quarter day here and there, February 29.

Chris - So relative to the sun, you are in the same spot on your birthday. But relative to everywhere else on the universe, you're not.

Andrew - After 365 and a quarter days, you're relative to the same spot with the Sun. but yeah, itís complicated.


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The shape of Earth's orbit is not constant due to the gravitational pull of other planets, see Ö

The position of the Sun is not fixed in space : it is part of the Milky Way galaxy which is rotating as it travels though space. RD, Fri, 16th Sep 2011

Yep, its always beneath your feet, unless you're standing on your hands, that is.

Earth takes 365ľ days (approx) to orbit the sun, so earth's position in relation to the sun will be slightly different and as RD wrote, the earth's orbit is quite the constant we imagine. Of course the position of the earth in relation to the other planets would be very different as you assume, since their orbits can take many years. (Jupiter takes roughly 12 earth years to orbit the sun).

As for the same position in the cosmos every year, no way. Our galaxy, the Milky Way, is expanding, the whole universe is expanding, so in a year from today, we will be a long way from where we are now. Don_1, Thu, 22nd Sep 2011

Isn't the same position in the universe a rather meaningless concept ?
the universe has no coordinate system ! syhprum, Mon, 26th Sep 2011

So, with the orbit around the sun, you would be within about Ī1į of the same place every year with the variability due to the leap years.

There are actually several ways to determine the position around the sun. 

Our years are based on the maximum and minimum angles of inclination of the earth with respect to the sun. with the Winter Solstice (North) being on December 21 or 22, and the Summer Solstice (North) being on June 20 or 21.  Spring Equinox (North) being on March 20 or 21, and the Fall Equinox (North) being on September 22 or 23.  And, perhaps a slight variation with the century and millennium adjustments to our calendar.

Another way to consider years is Earth's location with respect to the Milky Way, and the rest of the Universe.  This goes through a slow precession of about 26,000 years.  So, every century, the location of Earth on your birthday with respect to the stars in the Milky Way will change by about 1.38į.

The sun is also orbiting around the Milky Way, once every 250 million years or so.  So, you will have to wait another 250 million years for it to return to the same place in its orbit around the Milky Way.

CliffordK, Mon, 26th Sep 2011

I think you should ask forum this question.  Ask them if the plough will always be in the same position, altitude and compass bearing; in the night sky at 9pm GMT on your daughters birthday.  Then check it out!! Make sure they have got it right!!
CliveS acsinuk, Tue, 27th Sep 2011

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