# The Naked Scientists Forum

### Author Topic: Is Earth always in the same location on your birthday?  (Read 5580 times)

#### thedoc

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##### Is Earth always in the same location on your birthday?
« on: 11/07/2016 11:57:01 »
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?

Visit the webpage for the podcast in which this question is answered.

« Last Edit: 11/07/2016 11:57:01 by _system »

#### RD

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##### Is Earth always in the same location on your birthday?
« Reply #1 on: 16/09/2011 17:24:55 »
The shape of Earth's orbit is not constant due to the gravitational pull of other planets, see …
http://en.wikipedia.org/wiki/Milankovich_cycles

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.
http://en.wikipedia.org/wiki/Milky_way#Velocity
« Last Edit: 16/09/2011 17:29:51 by RD »

#### Don_1

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##### Is Earth always in the same location on your birthday?
« Reply #2 on: 22/09/2011 18:00:27 »
Yep, its always beneath your feet, unless you're standing on your hands, that is.

Earth takes 365¼ 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.

#### syhprum

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##### Is Earth always in the same location on your birthday?
« Reply #3 on: 26/09/2011 04:50:10 »
Isn't the same position in the universe a rather meaningless concept ?
the universe has no coordinate system !

#### CliffordK

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##### Is Earth always in the same location on your birthday?
« Reply #4 on: 26/09/2011 06:41:17 »
So, with the orbit around the sun, you would be within about ±1° 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 1.38°.

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.

#### acsinuk

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##### Is Earth always in the same location on your birthday?
« Reply #5 on: 27/09/2011 10:38:15 »
I think you should ask space.com forum this question.  Ask them if the plough [or dipper] 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

#### thedoc

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##### Hear the answer to this question on our show
« Reply #6 on: 11/07/2016 11:57:01 »
We discussed this question on our  show

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

[Transcript to follow]

Click to visit the show page for the podcast in which this question is answered. Alternatively, listen to the answer now or [download as MP3]
« Last Edit: 11/07/2016 11:57:01 by _system »

#### The Naked Scientists Forum

##### Hear the answer to this question on our show
« Reply #6 on: 11/07/2016 11:57:01 »