Questions answered

By Sherry Seethaler

SAN DIEGO UNION TRIBUNE
December 22, 2004

QUESTION: Why does the rule about using your watch's hour hand to point to the sun and then finding south (assuming you're in the northern hemisphere) between the hour hand and 12 o'clock work? How does this relate to sundials?

– Christopher Wu, San Francisco

ANSWER:The sun reaches its high point in the sky at astronomical noon – a moment also known as the meridian. (Hence ante meridian or a.m. and post meridian or p.m.) In the Northern Hemisphere, the sun is due south at the meridian because only between the Tropic of Cancer and the Tropic of Capricorn is the sun ever directly overhead.

Therefore at noon, the shadow cast by the shadow maker – the gnomon – of a sundial will point directly north. For a sundial to tell time, the noon mark must be oriented to true-celestial not magnetic-north.

As the earth rotates, the sun appears to move from east to west around the sky and the shadow cast by the gnomon will move clockwise 15 degrees per hour (360 degrees in 24 hours).

Think of your watch as a little sundial. If you line up the hour hand with a shadow cast by the sun, you can look to the 12 to find the north/south line. However, because 360 degrees on a watch corresponds to 12 hours, rather than 24, the north/south line runs through a point half way between the hour and the 12. This point will face north between 6 a.m. and 6 p.m., after which it will face south.

Even correcting for daylight-saving time, your watch is not a perfect measure of direction because it is set according to your time zone, but astronomical noon varies across a time zone. Also, because of the Earth's tilt on its axis and its elliptical orbit around the sun, successive astronomical noons are sometimes more and sometimes less than 24 hours apart, causing up to an additional quarter hour difference between watch and sun time.

Why does stuff that's left out in the sunshine bleach/fade?

– Marilyn Wilson

The ability of a pigment to absorb certain colors and reflect others depends on the nature of the chemical bonds between the atoms that constitute the pigment molecule. It is the sharing of electrons between atoms that bonds them together. Light from the sun, especially the higher energy ultraviolet radiation, excites electrons and can break the bonds in the pigment. Of course, this process is not limited to pigment molecules: Textiles, plastics, wood, etc. are also degraded over time as sunlight causes the chemical bonds in them to break.

copyright 2004 The San Diego Union Tribune