Context: Bengaluru witnessed a unique celestial phenomenon called ‘Zero Shadow Day’ on April 25. In this all the vertical objects in the city will not have a shadow for a short period of time.
What is Zero Shadow Day?
- It is a day on which the Sun does not cast a shadow of an object at solar noon, when the sun will be exactly at the zenith position (the highest point in the sky).
- The Zero Shadow Day is restricted to locations between the tropics, and so places north of Ranchi in India are out of it.
- For every point on Earth between the Tropic of Cancer and the Tropic of Capricorn, there are two Zero Shadow Days a year.
Why does a Zero Shadow Day happen?
- Movement of the Sun from south to north from winter solstice to summer solstice (Uttarayan)and back from north to south (Dakshinayan) happen because Earth’s rotation axis is tilted at an angle of roughly 23.5° to the axis of revolution around the Sun.
- The Sun’s location moves from 23.5°N to 23.5°S of Earth’s equator and back. All places whose latitude equals the angle between the Sun’s location and the equator on that day experience zero shadow day, with the shadow beneath an object at local noon.
- The Earth’s rotation axis is inclined at 23.5 degrees to the plane of its revolution around the Sun, which is why we have seasons. This also means that the Sun, in its highest point of the day, will move from 23.5 degrees south of the celestial equator to 23.5 degrees north of the equator (Uttarayan), and back again (Dakshinayan), in a year. The northern most and southern most points are the two solstices, and the crossing of the Sun across the equator are the two equinoxes.
What is an equinox?
- During the equinox, the sun crosses the plane of Earth’s equator, making nighttime and daytime (roughly) equal length all over the world.
- In the Northern Hemisphere, the spring equinox, or vernal equinox, occurs around March 21, when the sun moves northward across the celestial equator.
- The autumnal equinox occurs around September 22 or 23, when the sun crosses the celestial equator going south.
- In the Southern Hemisphere, it’s the reverse.
What is a solstice?
- A solstice is one of the two times of the year when the positioning and tilt of Earth relative to the sun results in the most amount of daylight time or the least amount of daylight time in a single day.
- Technically, a solstice is one of the two the exact moments in the year when the sun reaches its northernmost point (around June 21, when the North Pole tilts closest to the sun) or southernmost point (around December 22, during the winter solstice) from Earth’s equator.
- The solstices are traditionally considered to mark the start of summer and winter.
- In the Northern Hemisphere, the summer solstice occurs in June and the winter solstice occurs in December.
- In the Southern Hemisphere, it’s the opposite.