Q: Where and when on Earth can the Sun rise in the west?

A: Near one of the poles in spring, since the earth's motion round the sun is then more significant than its rotation.

The point is that in this situation the sun is not following an arc overhead from one horizon to the other as usual. During the winter the pole is in constant night, with the sun below the horizon. At the end of winter, for an observer near the pole the sun is going round and round the horizon (one circuit per day) but is always just below the horizon so is out of sight. As spring comes the sun's track gradually rises in the sky, so it moves from going round and round the horizon but just below it, to going round and round the horizon but just above it.

"Sunrise" occurs at the point where the sun's rising track moves
above the horizon. Consider a ring of observers standing around the
pole. They will all see this sunrise happen at the same place on the
horizon. However, the *direction* that they say that it happens
will depend on where they're standing relative to the pole. For example,
at the north pole an observer who happens to have the pole on their
right as they watch the sunrise will call the direction that they're
looking "west".

Another way to look at this:

Let `T` be the angle of tilt of the earth's axis (about 0.4
radians), and let `R` be its radius (about 6.4e6 m). Suppose
for the moment that the earth isn't rotating on its axis, and consider a
small region around one of the poles. As the earth moves round the sun
then in spring the night-day terminator will move across this region
with speed 2 pi `R` sin `T` / year, and if we stand in
the appropriate part of the region we will call the direction in which
the sun appears to be rising west.

Now suppose the earth *is* rotating on its axis. Then we will
be carried from west to east, ie in the same direction as the
terminator. However, suppose we are so close to the pole that we are
moving slower than the terminator - then the terminator will pass us in
the same direction as before, and we will still see the sun rise
(slowly) in the west. If our latitude is pi/2 - `L` then our
speed due to the rotation is 2 pi `R` sin `L` /
day. Thus - assuming that `L` is small relative to `T`
so that the terminator's speed is constant throughout the region - we
must have

2 pi `R` sin `L` / day < 2 pi
`R` sin `T` / year, ie sin `L` < sin
`T` day/year.

Our distance from the pole is `RL`. With the above values of
`T` and `R` we can therefore be up to 6.9 km from the
pole.

This page is maintained by Thomas Bending,
and was last modified on 30 December 2018.

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Copyright © Thomas Bending 2018.