# Astronomy/the orbit of earth

Question
Hi, I am in Australia, and I have been learning about the Earth's orbit around the Sun, and I noticed that when the Earth is on one side of the Sun, the equator seems to move downwards, and when Earth is on the opposite side of the Sun, the equator seems to move upwards. Doesn't that mean that the Sun should appear to go up then down in the sky for half the year, and down then up for the other half? I don't understand why the change of direction in the equator doesn't change the way the sun looks during the day. Thanks for your help!

Hello,

I think this previous answer can help to shed some insight,

http://en.allexperts.com/q/Astronomy-1360/2011/12/earth-planitary-science.htm

especially by reference to the diagram showing Earth at 4 different points in its orbit around the Sun (i.e. at the start of the different N. Hemisphere seasons)

As you will see, the orientation of the Earth's axis is fixed - but it maintains this fixed orientation throughout its orbit. Hence, much more sunlight is received at the summer solstice than the winter solstice. (Of course, these seasonal points, references are exactly reversed for the S. hemisphere).

The orientation for the axis, is also the orientation for the equator if you look carefully. Thus, the Sun is directly over the Tropic of Cancer (23 1/2 N latitude) on June 21 and over the Tropic of Capricorn (23 S latitude) on Dec. 21)

Hence, none of these reference circles on the Earth itself moves but rather their orientation with respect to the Sun alters because the Earth's axial tilt remains fixed in its orbit.

But you are correct in the sense that for half the year the Sun appears to move "up" (if by up you mean north of the winter solstice) and for half the year "down" - if by down you mean south of the summer solstice. Thus, the Sun was at its southernmost point on the winter solstice or Dec. 21 (-23.5 S) , and continues now moving north of that reference circle until it will reach a maximum northern lat. (23.5N)  on June 21, the summer solstice. (Again, reverse all these for the S. hemisphere)

After the summer solstice it will move south until reaching the equator on the autumnal equinox, then further south reaching the winter solstice on Dec. 21.

One more point here, the changing relations of the Sun due to the Earth's orbital path,  noted above,  refer to how it changes position during the *year* not the day.  If you can obtain a device to measure the Sun's *azimuth* (available to surveyors or at land surveyor schools) you will be able to track the Sun's seasonal changes along your horizon by reference to its changing azimuth.

Hope that this helps!

Astronomy

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#### Philip Stahl

##### Expertise

I have more than forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics pertaining to sunspot morphology and flare geo-effectiveness.

##### Experience

Astronomy: Worked at university observatory in college, doing astrographic measurements. Developed first ever astronomy curriculum for secondary schools in Caribbean. Gave workshops in astrophysics and astronomical measurements at Harry Bayley Observatory, Barbados. M.Phil. degree in Physics/Solar Physics and more than twenty years as researcher with discovery of SID flares. Developed of first ever consistent magnetic arcade model for solar flares incorporating energy dissipation and accumulation. Develop first ever loop solar flare model using double layers and incorporating cavity resonators.

Organizations
American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union.

Publications
Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'. 'Astronomy and Astrophysics: Notes, Problems and Solutions'.

Education/Credentials
B.A. Astronomy, M. Phil. Physics

Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research (1980), Barbados Astronomical Society Award for Service as Journal Editor (1977-91)

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