Ask A Scientist© Astronomy Archive

Author:      bess amaral
What would a "day" look like on Mercury?  on Uranus?  With respect to the
apparent path of the Sun to an observer on the surface of these planets...I
had read an article that the Sun would appear to make a retrograde movement
on Mercury...is that true?  Why?/Why not?  ...and which orientation does
Uranus take as it rotates on its side around the Sun (does it keep one pole
always facing away?)

Response #:  1 of 1
Author:      daniel n koury jr
An interesting question.  I will try to sketch out an answer, though I am
not entirely sure.  First imagine that the Earth rotated on it is axis at
the same rate it does, but in the opposite direction.  Then a "day" would be
the same length, but the Sun would "rise" in the west and "set" in the east
rather than rising in the east and setting in the west as we observe.  This
is how such things could be reversed.
Now, suppose that we could slow down the rotation so that the Earth rotated
on its axis in one year instead of one day.  Then the rotation would cancel
the motion of the Earth around the Sun, and the Sun would never rise or set
on a given spot.  One side would always be "day" and the other side always
"night".  This is the situation with the Moon, which always keeps the same
face towards the Earth.  That is why the Moon always looks the same to us.
Now let us consider Uranus.  It rotates on it is side, but this does not
really affect the way a day looks as long as Uranus rotates in much less
time than it takes for it to go around the Sun.  What will get really wacky
are the seasons.  A minor point here is that the "surface" of Uranus we see
is really the cloud tops, which do indeed rotate in much less than a "year"
on Uranus.  But there is nothing to stand on to watch the sunset there, and
no one really knows where the "surface" is if there is one.
Let me back up a bit.  A day is the time the planet takes to spin on it is
axis, like a top spinning.  A year is the time for a planet to go around the
Sun.  (You probably already knew this, but just in case.)
Now let us try Mercury.  This planet is in a resonance where it rotates in
2/3 the time it takes to go around the Sun.  Since it "spins" more slowly
than it goes around the Sun, the Sun will indeed "rise" in the west instead
of the east.  (Think about the case where the Sun "stands" still.  If
rotation is faster, then the Sun must move "forward" i.e. the way it does on
Earth.  If rotation is slower, the Sun must move backwards.)