Solar Month or saura māsa [सौर मास]

Solar Month is defined by the time taken by the Sun to transit from beginning of one zodiac sign/ rāśi to the succeeding one in the Zodiac Belt / bha-cakra [भ-चक्र], as seen from the Earth. It is actually the time taken by the Earth to go around the Sun.

The first Solar Month is considered to begin when Sun enters Aries/meśa rāśi [मेष राशि] i.e. enters at zero degree of Aries of the Sidereal Zodiac.

The 1st month, when Sun enters Aries/ meśa rāśi [मेष राशि] is named as caitra/चैत्र, which in the current times begins at around 15th of April.
The Sun would then enter Taurus/vṛṣabha rāśi [वृषभ राशि], and so on….until at the end of the 12 months it would have traversed the 12 zodiac signs and returned back to the zero degree of Aries/ meśa rāśi [मेष राशि].

The table below summarizes the transit of the Sun and the corresponding Months in Hindu and Gregorian Calendars:

Order	Sun enters	Vedic Name	Gregorian Month
I	Aries / meṣa (मेष)	caitra / चैत्र	Apr-May
II	Taurus / vṛṣa (वृष)	vaiśākha / वैशाख	May-Jun
III	Gemini / mithuna (मिथुन)	jyeṣṭha / ज्येष्ठ	Jun-Jul
IV	Cancer / karka (कर्क)	āṣāṛha / आषाढ़	Jul-Aug
V	Leo / siṁha (सिंह)	śrāvaṇa / श्रावण	Aug-Sep
VI	Virgo / kanyā (कन्या)	bhādrapada / भाद्रपद	Sep-Oct
VII	Libra / tulā (तुला)	aśvina / अश्विन	Oct-Nov
VIII	Scorpio / vṛṣcika (वृष्चिक)	kārtika / कार्तिक	Nov-Dec
IX	Sagittarius / dhanu (धनु)	mārgaśīrṣa / मार्गशीर्ष	Dec-Jan
X	Capricorn / makara (मकर)	pauṣa / पौष	Jan-Feb
XI	Aquarius / kumbha (कुम्भ)	māgha / माघ	Feb-Mar
XII	Pisces / mīna (मीन)	phālguna / फाल्गुन	Mar-Apr

The time taken by the Sun to transit respective zodiac signs is variable.
Thus, the period of the Solar Month varies slightly around 30 days (approximately 29-32 days).

The transit of Sun from one rāśi to another is termed as saṁkrānti [संक्रान्ति].
The Sankrantis are named according to the sign into which the Sun enters.
The most popularly known is the Makara-Sankranti, when the Sun enters Capricorn, since this is the beginning of the Uttarayana or the Northward/Upward journey of the Sun in the tilted Elliptic.

More about the Sankrantis and Ayanas of the Sun later…