Does the sunrise from the east and sets in the west exactly every day? Most of the people wake up way after the sunrise but some do love watching the sunrise, so wake up earlier. Are you the latter? If you are, then you must have noticed the shift in the Sun’s position on the horizon on a daily basis.
At least you might have observed that Sun rises (or sets) from a point in the horizon during winter whereas the point is different during summer. For example, my room (which faces the north direction and I live in northern hemisphere) receives no direct Sunlight during winter while it receives direct light from setting Sun during summer.
This observation led me to do some research on this shift of the Sun’s path in the sky and I found some interesting facts.
Why there is a shift in the Sun’s position on the horizon?
The thing that defines the trajectory of the Sun in the sky is the motion of Earth. So, this shift in the Sun’s position on the horizon or simply the change of the path of Sun in the sky depends upon the position of Earth relative to the Sun and the motion of Earth.
For a better understanding of it, we need a vivid insight of the relative motion of the Earth and the Sun. Finally, we need to know about the relative orientation of the Sun and the Earth.
The Earth – it’s position and motion
The Earth does not actually move in a circular path and its orbit is elliptical, with the Sun lying slightly away from the centre. So, during some time of a year, it is closest to the Sun whereas it lies farthest some other time.
The plane of Sun’s path in the sky (called the ecliptic), which is coplanar with Earth’s plane of revolution around Sun is tilted with respect to the equatorial plane of Earth. This angle of tilt is about 23.5 degrees. This is the main reason for the shift in the sun’s position on the horizon.
The tilt along with the revolution results in the variation in the view of the Sun and also the change in the season. But the most important fact is that the Earth’s axis always points towards the same point (ignoring the chandler wobble for short period of time). Due to this, the leftward tilt of the upper pole as in the figure is always leftward.
The tilt causes the rays of the Sun fall on the surface making an angle of 23.5 degrees with the vertical during the solstice whereas vertically during equinoxes. The angle at which the light rays fall is responsible for the view of it.
As the Earth moves from one solstice to the other, the trajectory has a declination of 47 degrees. This is due to the 23.5 degrees northward shift during one solstice from the equinox and 23.5 degrees southward shift during the other.
This signifies that for a person in the northern hemisphere, the Sun rises from slightly north of the east while sets slightly north of the west during summer. However, the trajectory is tilted southward. This is opposite for those living in the southern hemisphere.
Likewise during winter, for northern hemisphere, the Sun rises from slightly southward of east and sets slightly southward of west and opposite for southern hemisphere.
So, here we conclude the shift in the Sun’s position in the horizon is due to the tilt of the Earth’s axis and its unidirectional orientation (though the orientation shifts by 0.7 arcseconds every 14 months and is considered negligible). It would not otherwise.