Expert: Jayendra Upadhye - 7/14/2005

Question
At the time of the summer solstice, the sun is directly over the tropic of Cancer. In Ireland, the tropic of Cancer lies to the south; yet the sun appears to set in the north west and rise in the north east. Because the tropic of Cancer is south of Ireland, I would expect the sun to set in the south west and rise in the south east. Where am I going wrong.

Answer
Hi John,
Go directly to the sterisked (***) line towards the end if you want the answer directly, else read on..

You have to keep the following in mind when trying to find out why your observation is such.

For an observer at a particular latitude,
1 - his "Nadir" or Zenith, will point "up" along a line passing perpendicularly through the ground, into the sky. Perforce he will be always comparing "to the north" vs "to the south" with respect to the sun's location against this "plumb" line.

2 - The observer's "sky" is the observable space "above" a plane that is at right angles to this plumb.

3 - The day-night forms a circle around the earth, which is part of a hypothetical plane.

4 - When the earth's axis (containing the two poles) becomes coplanar with this day-night plane, we get the equinoxes.

5 - The 23.5 deg "tilt" is the angle between earth's equatorial plane and the earth's orbital plane.
   OR conversely, it is the angle the earth's rotational axis subtends with the axis of the earth's orbital plane.

Some strange things follow:-
1 - At the equator, the day is always 12 hours irrespective of what time of year it is!
2 - However as one goes away from the equator, the length od day changes depemding on "summer" or "winter".
3 - On the equinoxes, even at the poles, the "day" and "night" are 12 hours each!
4 - After the march equinox, the sun never sets at north pole! And never rises at the south pole!
   Opposite happens after the other equinox.
5 - One has 24 hour daylight at the 66.5th northern parallel on summer solstice day in june, and converse for the winter solstice in the southern hemisphere.

************* answer to your problem ***********

1 - For all territories to the north of the equator, After the march equinox, the sun ACTUALLY RISES to THE NORTH OF TRUE EAST, progresses westwards, AND SOUTHWARDS, reaching ZENITH AT MIDDAY, then progresses to the west, then north at SETTING TIME.

2 - This is true ON THE TROPIC OF CANCER AT SUMMER SOLSTICE DAY TOO, when the sun rises north of east, approaches exact 90 deg nadir at mid day, and goes to the north west to set.

3 - ON that day, at the 66.5th parallel, the sun "just about"rises at TRUE north, travels around the horizon thru east to south, reaches a 47 degree south nadir at exact midday, travels west, then "just about" sets again at north! Completing one circle in 24 hours!

4 - As one Nears the north pole in the mean time, ever since the march equinox, the sun is going around in tighter circles, around an ever rising nadir, never setting at all.
[that nadir is 23.5 degrees at the pole itself on summer solstice day]

5 - Exactly on the pole, The sun has been steadyly rising from the east since the march equinox, and reaches zenith of 23.5 degrees on summer solstice day. then it slowly sets over next 3 month and sets on the sept. equinox. Afull 6 month day + 6 month night. [describing a painfully slow spiral path to its nadir and back over 6 months!].

So for you in ireland, it is no wonder the sun rises and sets north of true east and west respectively.

Please do rate the answer if it has helped.

Jayen