How is the position of Surya and Chandra related to Amavasya and Poornima?

Question

As far as I understand, in Jyotisha, Amavasya and Poornima are being related to the positions of Surya (Sun) and Chandra (Moon). What is this relationship?

How do these positions change in between an Amavasya and Poornima?

Also, how are these positions correlated with Surya Grahana and Chandra Grahana?

Answer 1

Normally Surya changes it's position by 1 degree per day. As there are 30 degrees in a Rashi hence Surya changes it's Rashi in a month. Change in Surya Rashi also indicates change in Solar Month.

In Amavasya:

During the day of Amavasya Surya and Chandra are in same Rashi. Also there comes a time when they come in exactly same degree. For eg. In Baisakh Surya is in Mesha Rashi. This means In the Amavasya of Baisakh Chandra will also be in Mesha Rashi. Due to this person who is born in Baisakh Amavasya have Mesha Rashi. And by the same method Surya is in Vrish, Mithun, Karkat.... and so on in the month Jestha, Ashad, Shrawan and so on. Hence in the Amavasya of these month Chandra is in these Rashi.

In Purnima:

During the day of Purnima Surya and Chandra are 180 degrees apart each other. ie. They are 6 Rashis apart each other. For eg. As Surya is in Mesha in Baisakh, in Baisakh Purnima Chandra will be in 6 Rashi after Mesha ie (Mesha + 6 Rashi apart ) = Tula Rashi.. In the same way for other months also Surya and Chandra are 180 degrees apart during Purnima.

Eclipse case:

There is chance of lunar eclipse in every Poornima and solar eclipse in every Amavasya. But it doesn't happen. An eclipse does not happen at every new or full moon, because the plane of the orbit of the Moon around the Earth is tilted with respect to the plane of the orbit of the Earth around the Sun (the ecliptic): so as seen from the Earth, when the Moon is nearest to the Sun (new moon) or at largest distance (full moon), the three bodies usually are not exactly on the same line.

Answer 2

The ancients have observed that everyday the Luminaries (includes both the ☽|Moon & ☼|Sun) dance throughout the BhūMaṇḍala (i.e., Zodiac taken as sky) as Rādhā & Kṛṣṇa (or alternately, Kṛṣṇa and the Gopīs). Now, every soli-lunar distance of 12° span marks 1 Tithi | 1 lunation.

Amāvāsya: Moon's stay-period with the Sun

• When both luminaries occupies the same sign (space), where moon is exactly 12° behind the former i.e., at 348° from the sun, then Amāvāsya, which is the fifthteen Tithi, begins.
• From then on, as moon ingresses closer to Sun's proximity i.e., at 354° onwards, Moon goes into a stage of combustion due to its proximity to sūrya's orb.
• Then just after Amāvāsya, the next soli-lunar 12° span marks the first Tithi and, in jyotiṣa parlance it is known as Kṛṣṇa Pratipada. From Amāvāsya, the first half of the Kṛṣṇa Pratipada Tithi span i.e., in the first 6°, Moon is seen to be coming out of that combusted-state and thence, from 7° onwards such a Moon is said to be purified (pavamāna) for it is a no longer combusted avasthā.

Pūrṇimā: Moon's farthest-distance from the Sun

Conversely, whenever Moon becomes exactly 180° away i.e., at equidistant = vis-à-vis to the Sun — such distance is called Pūrṇimā, and as such moon gets full raśmi (rays) of the sun at such distance.

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To sum it all up, when Moon joins sun, it's Amāvāsya. On the other hand, when Moon is opposite to the sun, it's Pūrṇima.

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Grahaṇa Kāla|Eclipse

Most of the times, usually on Pūrṇimās and Amāvāsyas, because the earth, sun & moon don't align on the same axis, therefore no eclipse occur. The day they all align (assuming it's Pūrṇimā or Amāvāsya), it's an eclipse: solar or lunar. It simply means either of the luminaries is getting blocked due to them being aligned exactly on the same axis. A schematic (visual) representation (from geocentric viewpoint) is given below to ease things out:

• Sūrya Grahaṇa| 🌎 ☽ ☼ ⟶ Here, Sun gets blocked [by the moon], hence the name Sūrya Grahaṇa.
  
• Candra Grahaṇa| ☽ 🌎 ☼ ⟶ Here, Moon gets blocked [by the earth], hence the name Candra Grahaṇa.

Such knowledge were common among the ancients Vaidika ṛṣis and thus, they even have some Ṛk¹ composed in relation to this.

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SPIRITUAL TIPS|During Sūrya Grahaṇa, it's one of the most spiritual time to get in contact with the divine. As such, it is said that Śiva power is the greatest during eclipse. Hence, such time is conducive (and is encouraged in various śāstras, like the ones mentioned below) for dāna, spiritual practice etc.

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These astronomical technicalities as described above form part of the Tithi basics and, on the other hand Tithi, Karaṇa, etc., are parcels of the Pañcāṅga framework. Many classical jyotiṣa (Astronomical) granthas (literatures)² cover them all.

Footnotes

¹ ṚgVeda 5.040, Sūrya Maṇḍala

yat tvā sūrya svarbhānus
tamasā vidhyad āsuraḥ ।
akṣetra-vid yathā mughdho
bhuvanāni adīdhayuḥ ॥ 5.040.05

यत् त्वा सूर्य स्वर्भानुस्
तमसा विध्यद् आसुरः ।
अक्षेत्र-विद् यथा मुघ्धो
भुवनानि अदीधयुः ॥ ५.०४०.०५
Tr. O Sūrya, when Svarbhānu (Rāhu) had gulped you by his shadow, the whole world in that darkness moment had become so shocked that, it was as if they had gone strayed.. having no clue where to go.

yaṁ vai sūryaṃ svarbhānus
tamasā vidhyad āsuraḥ ।
atrayaḥ tam anu
avindan na ha anye aśaknuvan ॥ 05.040.9

यं वै सूर्यं स्वर्भानुस्
तमसा विध्यद् आसुरः ।
अत्रयः तम् अनु
अविन्दन् न ह अन्ये अशक्नुवन् ॥ ०५.०४०.९
Tr. The same You (i.e., that eclipsed-Sun) were later freed by Maharṣi Atri. He achieved a feat which nobody else could.

² Some Astronomical classical literatures by Indian (genius) astronomers are:

1. Brahma Sphuṭa Siddhānta
2. Gaṇita Yukti Bhāṣā of Jyeṣṭhadeva
3. Laghu Bhāskarīya of Bhāskarācārya I
4. Mahā Bhāskarīya of Bhāskarācārya I
5. Siddhānta Darpaṇa of Sāmanta Candraśekhara
6. Siddhānta Śiromaṇi of Bhāskarācārya 2
7. Sūrya Siddhānta of Maya (Yāmala Bhāskara Stream)
8. Tantra saṅgraha of Nīlakaṇṭha Somayājī
9. Vāṭeśvara Siddhānta
10. Āryabhaṭṭīya of Āryabhaṭṭa