सिद्धान्तशिरोमणि: गणिताध्याय (भास्कराचार्य - ग्रहगणित, मध्यमाधिकार व स्पष्टाधिकार सटीक)

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the course of a certain period were computed whereby a
correct estimate of a sidereal solar year was arrived at.
This period could also be checked simultaneously by obser-
ving the interval between the heliacal risings or settings of
a particular star of the Zodiac as well. Thus far, we have
seen how the sidereal periods of the Sun and the Moon
were determined very accurately. It may be noted that
these periods as determined by the Hindu astronomers
were correct to a good number of decimal places.
When once the Sun and the Moon were found to be
having eastward motion among stars, and when it was
discovered that there were other luminous bodies like the
Jupiter and Venus etc. moving among stars, it was
attempted to determine their sidereal periods. It must
have been done as follows. In the first place, it was
noticed that these other luminous bodies which were five in
number, namely Mars, Mercury, Jupiter, Venus and
Saturn, were found to be having retrograde motion also
unlike the Sun and the Moon. As these five bodies were
looking like stars they were named Tārā-grahas i.e. grahas
looking like stars. Also a distrinction could be drawn very
easily between Mercury and Venus on the one hand and
the other three on the other, for, the former were always
found oscillating about the Sun, never parting from him
through long distances. Thus during the course of a suffi-
ciently long interval, the geocentric sidereal periods of
Mercury and Venus coincided with that of the Sun. In
other words, it was taken that the geocentric sidereal
periods of Mercury and Venus also were taken to be an
year. This is clear from the statements made by all the
Siddhāntas that in a Kalpa of 4320000000 years the Sun, the
Mercury and Venus all the three make 4320000000 sidereal
revolutions. Then with respect to the other three planets
Mars, Jupiter and Saturn, it was noticed that they were
having a pre-dominantly longer period of direct motion,
though there was a retrograde motion for some time. This