# Question of the Week

B R Sitaram

Here are the answers to last month’s questions!

1. Our textbooks say that the spring equinox is on March 21. However, this year, the spring equinox was on March 20 (at 9:20 am). How come there is this difference?
Actually, the date of the equinoxes and solstices varies for a very simple reason: we pretend that the earth takes 365 days to go around the sun and once in four years slows down to 366 days. Nothing of the kind happens: Every year, the earth takes exactly the same number of days (slightly less than 365 ¼ days) to go around the sun. As a result, every year, we make an error of ¼ days, which accumulates to 1 day at the end of four years and is then made to 0 again due to the leap year. An error of ¼ of a year (which can accumulate to ¾ days) can change the actual date of the equinoxes. For example, if in a particular year the equinox was on March 21 at 3.30 am, then in the previous year, it would have been 6 hours earlier, i.e., 10:30 pm on March 20! In addition, the dates of the equinoxes and solstices also slowly change but that is a much slower change (about a day in every 70 years).

2. Which law or laws of physics are violated or can be violated by living organisms?
None whatsoever! Every object, living or non-living, has to obey the laws of physics! (And chemistry, of course!)

3. Why did the British Government announce an award for a method to measure the longitude of a ship, while no such award was announced or made for determining the latitude of the ship? (The longitude award was announced in 1714 but was never awarded; however, many persons got financial assistance in carrying out research to solve the problem.)
Finding the latitude is trivial: all you need to do is to look for the pole star or the sun at noon and find its altitude. Longitude is completely different. In a sense, 0 latitude is fixed: it is the equator, defined by the intersection of the earth with a plane perpendicular to the earth’s axis. Longitude, however, is arbitrary, to the extent that the 0 of longitude can be chosen arbitrarily. The only guaranteed way of finding longitude is to determine the time (e.g., of noon) at a location and know what the time is at 0 longitude. Say it is 15 minutes past noon at Greenwich when it is noon at your place, then you are 3.75 degrees west of Greenwich. So, you need an accurate chronometer, which will tell you the time in Greenwich, no matter where you are in the world!

4. Your friend’s daughter has just been to the moon on an exchange visit for a few months and has sent you a painting of the lunar sky with the sun at the centre. What is the most important novel feature that you will notice as soon as you look at the picture?
The most striking feature would be the sky, of course: it would be black! Also, you will be seeing stars all around the sun! Of course, there is no atmosphere on the moon and hence sunlight is never scattered: you can see sunlight only if you look directly at the sun! Stars would be visible, as sunlight would not interfere with your seeing them.