Sankranti: the Sun's entry into a sign
Sankranti is the moment the Sun enters a new sidereal rashi. There are twelve sankrantis in a year, one for each sign.
What it is
The Sun moves through each sidereal sign in about a month, so the sankranti dates shift slightly from year to year. Makar Sankranti marks the Sun's entry into Makara, which is the most widely observed one in India.
How it is worked out or read
The moment is found from the Sun's sidereal longitude, the same value used for the rashi of the Sun in the birth chart. The site does not yet list sankranti dates in the panchang.
Where it appears on this site
The Sun's sign is shown in the birth chart. The Makar Sankranti article explains why its date varies.
The moment the Sun changes sign
A sankranti is the moment the Sun enters a new sidereal sign. Each year has twelve sankrantis, one for each sign, and the Sun takes roughly a month to cross each one. The moment is set by the Sun's sidereal longitude, which crosses a multiple of thirty degrees.
Which sankranti is most observed
Makar Sankranti, the entry into Makara, is the most widely observed in India, and it falls in mid January. Other sankrantis are observed in different regions, sometimes with harvest or local customs attached. The sankranti is a solar event, and so its date is less tied to the lunar calendar than the tithi-based festivals are.
The date and the rule
Because the Sun's motion is not an exact number of days, the sankranti date can shift by a day from year to year. The moment of entry is what matters for the rule, and a reader should check whether a festival uses the moment, the sunrise after it, or the civil day in which it falls.
Sankranti and the solar calendar
Because sankrantis follow the Sun, they are the natural markers of the solar calendar within the sidereal zodiac. A reader who follows the twelve sankrantis across a year sees the Sun pass through all twelve signs, and can use them as a rough map of the solar year. Each sankranti also marks a change of the seasonal tone that the traditional calendar attaches to the signs.
Because the sankranti dates shift slightly from year to year, a reader who wants the exact moment should check an almanac for that year, rather than assume the date of the previous one.
Festivals connected with sankranti
Makar Sankranti is the best known, but other sankrantis are observed with local customs, often connected with the harvest and with the bathing, giving and gathering that mark the change. These observances are rooted in the agricultural year and in local life, and a reader can see them as a meeting of the solar calendar and the community's customs.
Sankranti and the month
In solar calendars used in some regions, each sankranti begins a new solar month, so the month names follow the Sun's sign directly.