The interval between two successive passages of I he Moon through the ascending node is called a nodical month. It is of 27.2122 mean solar days. It is smaller than the sidereal month (27.32 days) because during a month’s interval Rahu moves backwards on the /.odiac towards the Moon.
The interval required by the Moon to move from perigee to perigee is called the anomalistic month. Its duration is 27.5546 mean solar days.
When the longitudes of the Sun and the Moon become exactly equal the amavasya ends. The period between the ending of one amavasya to the end of next amavasya is called a lunar month. It is also called t he synodical month, i.e. 29.5306 mean solar days.
When the centre of the Sun enters from one rashi to another, it is the sankranti of the other rashi. The time taken by the Sun from one sankranti to another is called a solar month. The time interval of every solar month differs because the angular velocity of the Sun is not uniform. When the angular velocity is more, the Sun crosses one rashi or sign early and that solar month is smaller. Conversely, when the angular velocity of the Sun is less, the solar month is bigger. The … Continue reading
Lunar day or a tithi is the average time taken by the Moon from one tithi to the next tithi. Each tithi represents 12° phase difference between the Moon’s position from the Sun’s position, i.e. Moon’s advancement over the Sun by another 12°. This average time is 23 hours 37 minutes 28 seconds.
The time interval from one rising of a nakshatra to its rising next time is called a sidereal day or nakshatra din. It is of about 23 hours 56 minutes and 4 seconds in solar day hours.
Nowadays, the time from one midnight to another midnight is a civil day. According to our ancient system, the time interval between one sunrise to another sunrise is called one ‘savan day’ i.e. civil day = 24 solar time hours. Mean solar day = 24 hours 3 minutes 56.5 seconds in sidereal time hours, etc.
The semi-major axis of the Earth’s orbit is known as astronomical unit. In other words, the half of maximum distance + minimum distance of the Earth from the Sun is known as astronomical unit (AU). One AU = 930 lakh miles In other words, the distance between the Earth’s two positions at extreme points is two AUs. It can also be expressed as the mean distance of the Earth from the Sun.
Light travels at the rate of 1,86,000 miles (or 3,00,000 km) per second. The distance traversed by the light in one year is known as light year. light year = 3,00,000 x 60 x 60 x 24 x 365.25 km. So, one light year = 9.46 x 1012 km. i.e. 9,460 billion kilometres or 5,880 billion miles.
Three systems for measuring distance in space are in vogue. (1) light year (2) astronomical unit (3) parsec