All this time I thought that the sun's angular velocity along the epicycle was measured relative to a line from Earth the the sun's apogee (taking the anomalistic orbit as the reference frame, rather than the #zodiac or the #ecliptic), but it turns out it's actually measured relative to a line joining the centre of Earth to the centre of the sun.

#Astronomy #Jyotisha #HistoryOfAstronomy

#SuryaSiddhanta verses 3:9-12 describe the Precession of the Equinoxes, but assumes that the northward equinox osscilates around Revati rather than moving east (relative to the zodiac) at a constant rate.

But the numbers give, if read correctly, can be interpreted as giving a tropical year of 365 days 5 hours 50 minutes 41.7 seconds, which is less than 2 minutes longer than the actual tropical year.

#Jyotish #Jyotisha #Astronomy

The #SuryaSiddhanta's methods for calculating the positions of the sun and moon at times specify that the absolute value of the jya of an arc must be added to some value in some cases, and subtracted in others.

Since the jya is the predecessor to the modern sine, in this we see the earliest inklings of negative values of trigonometric functions. It's interesting how ideas develop.

#Jyotisha #Trigonometry #Maths #HistoryOfMaths #MathHistory

The first few verses of chapter 3 of the #SuryaSiddhanta is the only place in the book to use the Roman custom of taking the ecliptic as fixed and the zodiac as mobile; the entire rest of the siddhanta treats the zodiac as the fixed reference frame, and considers the ecliptic to just be a line along the zodiac.

#Jyotisha #IndianAstronomy