The sidereal year
The sidereal year is the time the Sun takes to return to the same position against the stars, about 365.256 days.
What it is
The sidereal year is slightly longer than the tropical year because of precession, which moves the equinox point backward. The difference is about 20 minutes a year.
How it is worked out or read
The sidereal year is the reference for the sidereal zodiac, and it is the year that the sidereal Sun's sign cycle follows.
Where it appears on this site
See tropical year and precession of the equinoxes.
The number itself
The sidereal year is the time the Sun takes to return to the same position against the background stars. Modern astronomy gives it as about 365.25636 days, or 365 days, 6 hours, 9 minutes and about 10 seconds. The tropical year, which measures the return of the Sun to the March equinox, is about 365.24219 days. The gap between the two is roughly 20 minutes a year, and that gap is the whole reason sidereal and tropical zodiacs drift apart.
The difference comes from the slow wobble of the Earth's axis, called precession. Because the equinox point moves backwards along the ecliptic by about 50.3 seconds of arc each year, the Sun reaches the equinox a little before it has completed a full circuit against the stars. Over about 71 to 72 years the difference adds up to one degree, and over the full precession cycle of roughly 25,800 years it adds up to the whole circle.
Why Jyotish cares about this year
Jyotish is a sidereal system: its signs and nakshatras are fixed against the stars, not against the equinox. So the year that matters for the Sun's passage through the rashis is the sidereal year. Each sankranti, the moment the Sun enters a new sidereal sign, arrives about 20 minutes later each year relative to the seasons. That is why Makar Sankranti, which once fell near the winter solstice, now falls around 14 or 15 January, more than three weeks after it.
The classical siddhantas used their own values for the year, very close to the modern sidereal figure. The Surya Siddhanta, for example, gives a year slightly longer than the modern value, and small differences of this kind accumulate over centuries. Modern panchang makers use current astronomical values, which is one reason their dates differ slightly from those computed by older methods.
How the site uses it
The calculators on this site do not count years at all. They compute the tropical position of the Sun for the exact moment from an astronomical model, then subtract the chosen ayanamsa to obtain the sidereal position. The ayanamsa itself grows each year by about the amount of precession, so the effect of the sidereal year is built in. A reader who checks the Sun's sidereal longitude on the same calendar date in two consecutive years will see it shift by a small fraction of a degree, which is the same 20-minute drift seen from a different angle.
A simple check a reader can do
Look up the time of Mesha Sankranti in a printed panchang for two years a decade apart. After allowing for leap years, the moment shifts later by roughly a few hours over the decade relative to the civil calendar. That steady drift is the sidereal year showing itself in an everyday almanac, and it is a good exercise for anyone who wants to see why the choice between tropical and sidereal frames is not a matter of taste but of definition.