### Re: what is the relative ground speed of a setting sun?

Date: Sun May 23 01:27:57 2004
Posted By: Martin Smith, Engineering, B.E., M.EngSc., Uni of Qld / airline pilot
Area of science: Physics
ID: 1085166350.Ph
Message:
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As the earth turns the sun traces a path across the sky.  The exact path it
traces depends upon your latitude, however the path over a day is
repetative. (It does change slowly from day to day, but for this question
we will ignore that change).

Now the path repeats over 24 hours because that is how long the earth takes
to spin around once.  To keep the sun in the one spot you need to go as
fast as the earth is turning.  So at the equator you would need to go the
circumference of the earth in one day.

At different latitudes the distance around the earth changes.

See here for an explanation

Quote
C = 2 pi r cos(x),

where pi = 3.14159..., r = the earth's equatorial radius = 6378 km, and x is
the angle of latitude.
End Quote

At the equator the distance is 40 0074 km. So you would need to be
travelling that distance in 24 hours.  That is 1670 km/hr.

At 23.5 degrees latitude the distance is 36 750 km.  that equates to a
speed of 1531 km/hr.

A 737-400 has a true air speed around about 430 nm/hr or 796 km/hr, a
747-400 does about 500 nm/hr which is 926 km/hr.

Martin Smith

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