Solstice/equinox dates for Saturn?
Posted 30 January 2012 - 05:39 AM
Posted 01 February 2012 - 02:20 PM
Anybody know where I can find this? I plan to use this data to interpolate the "cross-quarters" in the Saturnian year, which lie halfway between the solstices and equinoxes (in other words, when the RA of the Sun as seen from Saturn is 3/9/15/21 hours or 45/135/225/315 degrees). I believe the late spring cross-quarter (equivalent of early May or "Beltane" on Earth) occurs in 2013, which marks the beginning of northern "Cassini Season" when the rings are really open from the north.
For equinoxes and solstices using RA may be good enough, if you mean measured not from the Earth but from the Sun or Saturn. Better would be 0/90/180/270° of Saturn’s heliocentric longitude as measured on Saturn’s ecliptic. In the cases of the cross-quarters, utilizing RA leads to severe distortions, since RA takes into account Earth’s axial tilt of 23.44° (or 26.75° if using a coordinate system based on Saturn’s axial tilt).
What you want is 45/135/225/315° of Saturn’s heliocentric longitude as measured on Saturn’s ecliptic. That’s rather tricky. If we consider the Earth’s ecliptic to be close enough, then I’ll give you a hint for doing the math.
The following formula from Jean Meeus’ Astronomical Algorithms gives the heliocentric longitude of the ascending node of Saturn’s ring plane (and equator) on Earth’s ecliptic. When Saturn is at that heliocentric longitude, it is near its own “vernal equinox”.
169.5084° + 1.394681° T + 0.000412° T²
T is the time in centuries since 2000 JAN 01 at 12:00 TD (Dynamical Time, but UT is close enough). Saturn is currently at heliocentric longitude 203.8°. Saturn’s own “vernal equinox” last occurred on 2009 AUG 10.
For my graph of Saturn’s ring tilt and related graphics, see my Saturn webpage at www.CurtRenz.com/saturn
Posted 01 February 2012 - 03:35 PM
2009 AUG 10 – Vernal Equinox
2013 MAY 16 – Spring Cross-Quarter
2017 MAY 24 – Summer Solstice
2021 JUN 20 – Summer Cross-Quarter
2025 MAY 05 – Autumnal Equinox
2028 DEC 02 – Autumn Cross-Quarter
2032 APR 11 – Winter Solstice
2035 AUG 04 – Winter Cross-Quarter
2039 JAN 22 – Vernal Equinox