14 April 2011

Sun Doesn't Rise Exactly in the East Always

We have been hearing from childhood that sun rises in the East. But how many of us know that sun doesn't always rise in the East?
Actually only twice in an year does sun rise in the East. On all other days, the position where it rises will be either Northward or Southward to the exact East. And what is the reason? It lies in the fact that the axis of earth's rotation is not perpendicular to the plane in which it revolves around Sun. It is inclined at an angle of 23.5 degrees to its orbital plane.

See the figure above.It shows the variation in the circle of illumination (the circular portion on earth illuminated by sunlight) at different positions of earth on its orbit. It can be seen that on march 21 and September 22, sunrays will fall vertically at the equator. These days are called Vernal equinox and Autumnal equinox respectively. On these days, day and night will be of equal duration in equatorial region, i.e. zero degree latitude. Similarly, in other latitudes, there will be two days(separated by six months) in an year having equal duration of day time and night time and on these days sun will be rising exactly in the East. That means, Sun doesn't rise exactly in the east on days other than equinoxes.
This can be understood more simply by considering the example of a vertical post and its shadow. If we observe the shadow of the post on morning, it will be along west. As time passes its length decreases and after noon it increses along east. But does its length become zero at noon? Not always. Only on equinoxes, we can see that. On noon of all other days, shadow will be seen along north or south directions, although small in length.


  1. Ok. This is really splitting hairs here. The bottom line is that the sun still rises generally in the east although it may be a few degrees northward or southward. This does make the writer look really smart though, so mission accomplished.

  2. Actually this is critical if you want to determine direction by the sun with some precision. It does not rise in the east, it is in the east at 6am, south at 12pm, and in the west at 6pm (assuming you correct for daylight savings time and you are more or less in the middle of your time zone in the N hemisphere)