Sunrise to sunrise: the synodic day, or null where there is not one.
1/T_solar = 1/T_sidereal − 1/T_year, with the sign of the rotation kept — a retrograde spin adds rather than subtracts, which is why Venus's solar day (117 d) is shorter than its year while its sidereal day (243 d) is longer.
year is the period about the star, not about whatever the body
immediately orbits. For a planet the two are the same and the distinction
is invisible. For a moon they are not, and using the wrong one is not a small
error: Luna is tidally locked, so its rotation period and its month are equal
and the difference of their reciprocals is zero — an infinitely long day,
for a body whose sunrises are 29.5 days apart. What actually moves the Sun
across a moon's sky is the planet's year, and against Earth's 365.25 days the
same subtraction gives the synodic month exactly.
Null when the two rates cancel: a body turning once per year keeps one face to its star and has no sunrise at all. Mercury does not — it is in a 3:2 resonance, which is why its solar day is 176 days rather than infinite.
- rotationnumber
- Undocumented
- yearnumber
- Undocumented