Astronomy/Earth's rotation


Dear Tom, I know this is a very silly question, but could you please tell me if it is just the tilt of the Earth's rotation axis that makes the Sun and Moon appear to trace a semi circle in the sky? Thanks!

Hi Elisabeth,
The semi-circle we witness is due primarily to the Earth's daily rotation on its axis, and your location on the globe, primarily your latitude. Observers at the equator see the sun and moon rise due east, go through their zenith (the overhead point) and set due west.  Observers at the poles see the sun and moon completely circle their horizon line. (They can see the entire "circle" in their sky because there is no EAST/WEST horizon blocking their view!) Remember, every direction from the South Pole is north, and the reverse is true at the North Pole!)
The 23.5 degree tilt causes the "semi-circle in our sky"... to move from north to south and from south to north during the yearly period, by 47 degrees or 23.5 doubled. That's why, from Australia, in your winter months of June and July, the sun and moon track low in the northern sky, and during your summer months of December and January, the sun and moon semi-circle cross much higher (by 47 degrees) in your summer sky.
The reason it's only a semi-circle is because your eastern and western horizons get in the way.  If the Earth was a small dot or point source, then all observers everywhere would see nearly the complete circle as there would be no EAST/WEST horizons to block the view of the Earth's rotation (and thus the sky's "apparent rotation") around us.
So the Earth's tilt of 23.5 degrees only makes the semi-circle climb and descend throughout the year, and it's primarily the rotation of the Earth, and your latitude location on the Earth that makes the "semi-circle".  IF the Earth's axis were NOT tilted, but straight up and down perpendicular to the plane of the Solar System (the ecliptic), then your own "semi-circle" path in the sky would remain constant, in the same position, no matter what season or time of the year.  So there would be no seasonal changes anywhere on the earth. Thus the  23.5 degree tilt just causes our seasons of winter, summer, spring, and autumn.
Hope this helps and makes sense,
Clear Skies,
Tom Whiting
Erie, PA USA  


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Tom Whiting


Astronomy has been my hobby/pasttime for over 50 years.  Currently own 3 telescopes, the largest of which is a 30 inch Newtonian truss Dob that is portable.I taught Astronomy/Meteorology at the University Level for 13 years before retiring in 1995. Being retired and home most of the time, I am able to answer all questions relatively quickly, unless it's a new moon weekend with good observing conditions.  No astrology questions please, or questions about alleged UFO picture identifications.


Experience: Astronomy has been my hobby and study for over 50 years. We currently now own a 30 inch portable telescope (Updated - Pennsylvania`s largest portable telescope). It can be seen on our website at: and also attend several regional starparties during the year, and have been on 5 total solar eclipse expeditions.

Organizations: President, Erie County Mobile Observers Group for over 15 years.

Publications: Wrote the "Over Erie Skies" newspaper article in our local newspaper for 11 years (1975-86).

Education: Masters Degree- Taught at the University level for 13 years. Retired 20 years -USAF Pilot - KC-135 with 180 combat missions;  Also Eagle Scout, Philmont staff 2 Yrs, Order of Arrow Lodge Chief, Ham Radio (inactive).

Awards: two discoveries: The mini-coathanger asterism in Ursa Minor (the little dipper) And the mini-ladle- another asterism in the bowl of Ursa Minor. Clients: Currently President of the ECMOG as mentioned above.

BS  Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PA Also retired USAF pilot, 20 years.

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