Astrology

Although the Sun is a star, but for astrological purposes it is considered as a planet.
What is the difference between a star and a planet?
Star has its own light, while planet takes light from others.
Another difference is that star seems static , while planet seems moving in relation to stars.

According to Gregorian calendar, the Sun moves from one Sign to another Sign every month  between the dates 13 and 16; and that day is called Sankraanti or the Transit day

7+2 Planets
There are 7 visible planets and 2 invisible planets which affect our lives, around us. They are - The Sun, The Moon, Mars, Mercury, Jupiter, Venus and Saturn; and the two invisible planets - Raahu (Dragon's Head) and Ketu (dragon's Tail). Since these two planets are invisible they are called Shadow Planets.

Retrograde Planet
Normally all planets move forward - towards East, but sometimes they go towards West too (except the Sun and the Moon. When they move towards the West, they are called retrograde (Vakree). It may be understood in the light of car or train travel - some trees seem going backward and some forward. In the same way it happens here too. Eventually all planets move towards East.

12 Signs in a Zodiac Band
The astronomers have made an imaginary zodiac band around the Earth and have divided it in 12 parts. These parts are called Signs. They are named on the names of the main constellations therein. They are - (1) Aries, (2) Taurus, (3) Gemini, (4) Cancer, (5)   (6) Libra, (7) Sagittarius,  (8)    (9) Scorpio, (10) Aquarius, (11) Capricorn, (12) Pisces. You can see only 6 Signs at any given time, because the other 6 Signs are on the other side, and because of the Earth being in between we will not be able to see them.

27 Nakshatra, or Asterism or Lunar Mansions
There are 27 Asterisms divided in 12 Signs - 2 and 1/4 Asterism per Sign.

If the Capricorn rises on Western horizon, then Aquarius will be a little bit up, then Pisces, then Aries.... Like this they will be moving towards East side. When Capricorn will set on Western side, Cancer will rise in East. Thus every 2 hours the next Sign appears on the Eastern horizon and will set on Western horizon in 12 hours.

21 March - day and night are of equal length - Sun is at Equator
22 June - the Longest day - Sun is at Cancer Line
23 September - day and night are of equal length - Sun is at Equator
23 December - the shortest day - Sun is at Capricorn Line
This is true only for Northern Hemisphere. For Southern Hemisphere, you have to reverse the dates.