| MadSci Network: Astronomy |
Hello Mr. Davis,
The reason you had such a hard time finding an answer to the question is because there is no simple answer. The Sun's path through sky and how much it changes daily and annually depends on both your location on Earth and the time of the year (location of Earth in its orbit about the Sun). The reason that the Sun travels a different path through the sky and rises and sets in a different spot each day is because of the tilt of Earth's rotation axis with respect to the plane of its orbit about the Sun.
If Earth's axis had no tilt (was perpendicular to the orbital plane) the Sun would travel the same path through the sky, rise directly due East, and set directly due West each day without change. The Sun's path through the sky would however be a different one depending on where on Earth you lived. If you lived at the equator it would pass directly overhead (through the zenith) everyday. If you lived at a latitude of 37° North the Sun would be 37° south of zenith at it's highest point (solar noon), or 90° - 37° = 53° above due South, everyday all year long. And if you lived at either the North or South Pole the Sun would always be right on the horizon; never rising or setting, just endlessly circling the horizon every 24 hours. In this simplest of geometries the length of the day would be the same everywhere on Earth because the Sun rises due East and sets due West as seen from everywhere. Earth would experience 12 hour days and 12 hour nights all year long. There would be no seasons.
But, Earth's axis is tilted 23.5° from perpendicular. There are only two days of the year when this tilt does not make a difference, the equinoxes. On these days Earth's axis is pointed parallel with the direction it is heading. Or put another way, these are the two days when the axis is pointed neither toward or away from the Sun, but perpendicular to it. The typical dates for the equinoxes are March 21st and September 21st. If you watch the Sun on these days it will rise exactly due East, reach its highest point (known as transit and solar noon) exactly 6 hours later and then set exactly due West another 6 hours later. This is the case everywhere on Earth (except the poles where again it just circles the horizon) on these days.
There are two more special places in Earth's orbit worth noting, the solstices. Theses are the days when the Sun reaches its extremes. It depends on whether you live in the northern or southern hemisphere, but I'll explain things from the northern hemisphere perspective and to visualize the southern hemisphere perspective just interchange the words 'north' and 'south' in the following discussion. The summer solstice occurs when the Sun rises and sets from it most extreme northern point on the horizon. The path it takes through the sky that day is the longest and these two things put together result in this being the longest day of the year. On this day the Sun never sets as seen from the North Pole (it stays at 23.5° above the horizon making a big circle in the sky), and it never rises as seen from the South Pole. It passes directly overhead at noon as seen from a latitude of 23.5° North, also known as the tropic of Cancer (the tropic of Capricorn in the southern hemisphere). The date of the Summer Solstice is typically June 21st. On this day Earth's axis is pointed toward the Sun. The Winter Solstice occurs 6 months later, around December 21st, when the axis is pointed away from the Sun. This is the shortest day of the year and the Sun rises from it most southernly extreme and sets likewise.
So, to try and generally answer your questions: The position on the horizon of the setting Sun will change the fastest and the most around the time of the equinoxes and it will change the slowest and the least around the times of the solstices. Likewise for the rising position, the path length through the sky and hence the length of day. The variation in the Sun's path, rising and setting positions, and length of day is greatest at the poles and the least at the equator. The poles essentially experience one 6-month day and one 6-month night per year. At the equator the longest day is 12 hours 8 minutes, the shortest day is 12 hours 6 minutes. So there is essentially no change in the length of the day at the equator. Everywhere else is between the extremes.
That's a lot of information, I know, and it takes a great deal of visualizing the 3-dimensional arrangement of Sun and Earth throughout the year. I'll recommend a few web sites with figures that might help with the visualization. Also getting out a globe and using any light source as the Sun could be a useful and fun thing to do with your son to model these geometrical arrangements. If you'd like to know when and where the Sun will set (or rise) on any given day as seen from where you live on Earth you can go to the US Naval Observatory's web site. They have a program there where you can enter your location and find out all that information including moon rising and setting times (another explanation altogether). There are also commercial computer programs that can show you that information visually as well (The Sky, Starry Night, Voyager, etc.). Finally, the most instructive way (for you and your son) to understand this phenomenon would be to observe it for a year. Take photographs or draw pictures of the sunset every couple days all year long and you'll be able to see just how much the Sun's position changes for yourself. You'll also notice how the length of the day changes.
Web Sites with good figures for understanding the Sun's path through the sky:
I hope this helps. Keep watching the sky.
-Bryan Méndez, UC Berkeley (bmendez@astro.berkeley.edu)
Try the links in the MadSci Library for more information on Astronomy.