Geodesy and Celestial Mechanics

The troposphere is the lowest layer of Earth's atmosphere, extending from the surface up to about 6-20 kilometers altitude. It contains approximately 75% of the atmosphere's mass and virtually all weather phenomena. The stratosphere, mesosphere, and ionosphere are progressively higher layers farther from Earth's surface.

A light year is a unit of distance equal to the distance light travels in vacuum in one year, approximately 9.46 trillion kilometers. It is commonly used to measure astronomical distances between stars and galaxies.

2. Overhead as seen from the north pole

Mercury has a very slow rotation and a fast orbit, leading to a solar day (sunrise to sunrise) that lasts about 176 Earth days. This is significantly longer than its sidereal rotation period of approximately 59 Earth days.

A light year is the distance that light travels in a vacuum in one Julian year. Despite having the word 'year' in its name, it is a unit of astronomical distance, not time. It is approximately 9.46 trillion kilometers.

The Earth's rotational speed is highest at the equator because it has the largest circumference to cover in a 24-hour period. As you move toward the poles, the distance traveled in one rotation decreases, thus the speed decreases.

Currently, glaciers and ice sheets cover about 10 percent of the Earth's total land area. Most of this ice is located in Antarctica and Greenland, though glaciers are found on every continent except Australia. During the last ice age, this coverage was much higher.

The average distance between the Earth and the Sun is approximately 149.6 million kilometers. In the Indian numbering system, 150 million is expressed as 15 crore.

The equatorial circumference of the Earth is approximately 40,075 km, which is commonly rounded to 40,000 km.

The thermal equator is the belt encircling the Earth where the mean temperature is the highest. Due to the distribution of land and sea, it is generally located slightly north of the geographical equator.

The Earth is not a perfect sphere; it is slightly flattened at the poles and bulging at the equator due to its rotation. This specific shape is called an oblate spheroid or a geoid. While it looks like a sphere from a distance, the oblate spheroid is the more precise geometric description.

This is the correct option. One metre is ten millionth of the distance from the equator to the north pole along the longitude passing through Paris in France.

Halley's Comet has an orbital period of approximately 76 years. Having last appeared in 1986, it is calculated to return to the inner solar system in mid-2061 to early 2062.