Aug 1, 20185 min read

Indian Astronomy: Part 2

Uttarayana and Dakshinayana

The movement of the sun along the ecliptic from its most northerly declination of +23.5 ° to its most southerly declination of ‑23.5 ° is called the Dakshinayana, or the southern movement of the sun. (summer solstice to winter solstice)

The movement of the sun along the ecliptic from its most southerly declination of ‑23.5 °to its most northerly declination of +23.5 ° is called Uttarayana, or the northern movement of the sun. (winter solstice to summer solstice)

Fig. 13.1 – Uttarayana and Dakshinaya

Uttarayana and Dakshinayana can be observed from the position of the sunrise on the horizon on different days of the year.

Fig. 13.2 – Sunrise on different days of the year

Rashis and nakshatras

The Zodiac or Rashi chakra and the nakshatras are the projection of the distant stars, which appear fixed with respect to the earth, onto the ecliptic.

Fig. 14 – Rashis or Zodiac constellations projected onto the ecliptic

The rashis divide the ecliptic of 360° into 12 regions of 30° . The sun spends about a month in each rashi, completing 360° in one year. Hence, the rashis can also be looked at as solar constellations.

The 12 rashis are given below.

The nakshatras divide the ecliptic of 360° into 27 regions of 13° 20’. The moon spends one day in each nakshatra, completing 360° of the zodiac in 27 days, which is the lunar month (sidereal*). Hence, the nakshatras can also be looked at as lunar constellations.

Each nakshatra is further divided into 4 padas (quarter) 3° 20’ each. Thus there are 27 * 4 = 108 padas.

The 27 nakshatras are given below.

* There are two ways the lunar month is reckoned : sidereal and synodic. We will look at it in Part 2 of this article.

Fig. 15 – Rashis and nakshatras

Orientation to the Galactic center

In Vedic astronomy, the rashi chakra is oriented to the center of our galaxy, the Milky Way. The light from the galactic center comes to the earth through the fixed stars of the constellation Sagittarius, Dhanu.

In the rashi chakra, the galactic center is located in the early portion of Sagittarius. The nakshatra Moola, meaning “root” or “source” suggests that it is the first of the series of nakshatras on a cosmic level. It marks the first 13 ° 20′ of Sagittarius, in the middle of which (6 ° 40′) is located the galactic center.

The previous nakshatra is called Jyeshta, meaning “the eldest” which marks the end of Scorpius. This shows that the ancients knew of the galactic center and named their constellations in such a way as to acknowledge it as the beginning.

Modern astronomy confirms this fact. Astronomers have discovered that the galactic center is an intense radio source known as Sagittarius A*. It lies in the direction of Sagittarius constellation, near the border with Scorpius (Fig. 18). Sagittarius A* is the most plausible candidate for the location of the supermassive black hole at the centre of our galaxy. (Fig. 19)

The Milky Way has 2 major arms and a number of minor arms. One of the minor arms, known as the Orion Arm, contains the sun and the solar system. The Orion arm is located between two bigger arms, Perseus (major arm) and Sagittarius. The sun is at a distance of 26,000 light-years from the galactic center. (Fig. 15)

The Milky Way galaxy can be observed from the earth as a beautiful stretch, because the plane of the solar system (ecliptic) is inclined at an angle of 60° to the galactic plane. (As shown in Fig. 16)

Fig. 15 – The location of our solar system in the Milky Way

Fig. 16 – Plane of the solar system (plane of ecliptic) and galactic plane

Fig. 17 – Milky Way galaxy as viewed from the Earth

Fig. 18 – Galactic center – between Sagittarius and Scorpio

Fig. 19 – Sagittarius A* at galactic center

The phases of the moon and thithi

Phases of the moon occur as a result of the position of the moon at different points in its orbit around the earth. What we refer to as phase is the part of the moon’s surface illumined by the sun, as seen from the earth. This is shown in Fig. 20.

Fig. 20 – Thithis of the moon

The waxing phase is called Shukla paksha and the waning phase is called Krishna paksha.

A thithi is 12°movement of the moon with respect to the sun in the rashi chakra.

A thithi is a lunar day. There are 15 thithis in the Shukla paksha and 15 thithis in the Krishna paksha.

Hence, 30 x 12°= 360° . The moon traverses the rashi chakra in 30 lunar days or thithis.

1.Pratipada

2.Dwitiya

3.Tritiya

4.Chaturthi

5.Panchami

6.Shashti

7.Saptami

8.Ashtami (Half moon)

9.Navami

10.Dashami

11.Ekadashi

12.Dwadashi

13.Trayodashi

14.Chaturdashi

15.Purnima (Full Moon in Shukla paksha) or Amavasya (New Moon in Krishna paksha)

Fig. 21 – Thithi – 12° movement of the moon with respect to the sun

Grahana – solar and lunar eclipse

The moon’s orbit path is not in line with the earth-sun orbit path. It is tilted by 5°. The points of intersection of the moon’s path and the sun’s path are called nodes. As the earth moves in its orbit, these nodes line up with the sun twice a year.

If the moon is between the sun and the earth, we get a solar eclipse, surya grahana.

If the earth is between the sun and the moon, we get a lunar eclipse, chandra grahana.

Total eclipses are possible only because the sun and the moon appear to be of the same size from earth. While the sun is 400 times bigger than the moon, it is also 400 times farther away.

Fig. 22 – Lunar nodes – intersection of moon path and sun path

Fig. 23 – Solar eclipse

Fig. 23.1 – Solar eclipse as seen from the earth

Fig. 24 – Lunar eclipse

Fig. 24.1 – Lunar eclipse as seen from the earth

The Navagrahas

The primary celestial bodies in Vedic astronomy are the navagrahas – 7 planets and 2 lunar nodes.

Since the orbits of all planets are nearly on the same plane as the ecliptic, with only minor differences, the grahas also traverse the rashi chakra.

The 7 planets:

1) Surya (Sun)

2) Chandra (Moon)

3) Budha (Mercury)

4) Shukra (Venus)

5) Mangala (Mars)

6) Guru (Jupiter)

7) Shani (Saturn)

Lunar nodes:

8) Rahu (Ascending node) ‑the node where lunar eclipse occurs

9) Ketu (Descending node) – the node where solar eclipse occurs

Fig. 25 – Grahas

Each graha has its own speed and corresponding time period of traversing the rashi chakra.

For example, the sun covers about 1° in 1 day. Thus, it traverses a rashi (30°) in about 30 days, which is one solar month. The sun goes through the entire rashi chakra (360°) in about 360 days, which is one solar year.

The moon covers 1° in 1 3/4 hours. It traverses a rashi (30°) in 2 and 1/4 days. It completes the entire rashi chakra in 27 and 1/3 days, which is also its period of revolution around the earth. This constitutes a lunar month.

Precession of the Equinoxes

The axis of the earth spins like a top, and its direction moves from the North star, Polaris, to Vega. As a result, the equinoxes (which are the intersection points of the ecliptic and the celestial equator) precess slowly westwards relative to the fixed stars, completing one revolution in about 24,000 years (According to Sri Yukteshwar*).

*Sri Yukteshwar Giri was the Guru of Paramahamsa Yogananda. His Guru was Lahiri Mahasaya, whose Guru was Kriya Babaji.

Fig. 26 – Precession of the equinoxes

Due to precession, the equinoxes have moved around the ecliptic such that the background constellation for the vernal point is now Pisces, and for the autumnal point is now Virgo.