Pune: Astronomy, to many people, may seem like a subject far removed from their daily lives. When children ask us how we know so much about the universe, we usually shrug our shoulders. However, using simple means, young people can deduce many facts that we learn in school about Earth and the way it moves through space.
The history of measuring time and navigation is pretty much the history of astronomy. How did we first figure out how to tell time, determine the cardinal directions at any given place, measure latitude, how detect the change of seasons and the tilt of Earth’s rotation? Can we find out for ourselves, at home or at school, the way it was done centuries ago by different civilisations?
This is the aim of the Zero Shadow Day (ZSD) campaign of the public outreach and education committee of the Astronomical Society of India. The committee wants to use ZSD to teach schoolchildren to answer these questions. Its website has collected all the relevant info and recently released an app as well (available in many Indian languages).
Zero shadow days, solstices and equinoxes
South-facing windows get direct sunlight in winter. The Sun doesn’t rise or set at the same point every day. In north India, there are fewer hours of daylight during winters.
These phenomena happen because of Earth’s tilted axis of rotation. This axis is an imaginary line around which the planet rotates. The equator is perpendicular to it – and also inclined at 23.5º against the plane of Earth’s orbit around the Sun. Hence, our rotation axis is also tilted at this angle relative to Earth’s axis of revolution. As Earth completes one rotation, all stars (including the Sun) seem to rotate around us daily in paths parallel to our equator projected on the sky.
However, because of axis’s tilt, the Sun’s daily path moves northwards and southwards over the course of a year. Because the tilt angle is 23.5°, the southernmost direct rays of the Sun, on winter solstice, fall 23.5° south of the equator. So on this day, the Sun will be directly overhead at noon for places that lie on the Tropic of Capricorn, at 23.5° S latitude.
Similarly, on summer solstice, the Sun’s rays fall perpendicularly onto Earth’s surface at 23.5° N, the Tropic of Cancer. During spring and autumn equinoxes on March 20 and September 23, respectively, the the Sun’s rays seem to fall straight down along the equator.
On days when the Sun is not directly overhead, all vertical objects will cast shadows even when it’s noon locally. In the same vein, for all places on Earth between the Tropics of Cancer and Capricorn, the Sun will be directly overhead at noon twice a year. These are the ZSDs for that location.
The ZSDs go northward till 21 June and then turn southward after. For example, ZSDs occur on April 15 and August 27 in Kochi; on April 24 and August 18 for Chennai and Bengaluru; on May 9 and August 3 for Hyderabad; on May 15 and July 28 for Mumbai; on June 5 and July 7 for Kolkata; and on June 8 and July 4 for Indore.
Such astronomical curiosities can be deduced using a stick.
Plant a stick vertically into level ground and mark the tip of its shadow every hour or so. You’ll notice that the shadow grows taller and shorter in time. This way, students can determine the four cardinal directions and the precise time of local noon. Repeat this exercise for a few days before and after a ZSD for your location and you’ll see the length of the shortest shadow decreasing to zero and increasing again.
In fact, by measuring the length of shadows from two places at noon, the Greek astronomer Eratosthenes was able to determine Earth’s diameter within a few percentage points more than 2,200 years ago.
Niruj Mohan Ramanujam is a radio astronomer and a member of the Public Outreach and Education Committee of the Astronomical Society of India and tweets at @NirujMohan
This article was originally published on India Science Wire.