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Erasmus Reinhold used Copernicus' theory to compute the Prutenic Tables in 1551, and gave a tropical year length of 365 solar days, 5 hours, 55 minutes, 58 seconds (365.24720 days), based on the length of a sidereal year and the presumed rate of precession. This was actually less accurate than the earlier value of the Alfonsine Tables.
J transit is the Julian date for the local true solar transit (or solar noon). 2451545.0 is noon of the equivalent Julian year reference. 0.0053 sin M − 0.0069 sin ( 2 λ ) {\displaystyle 0.0053\sin M-0.0069\sin \left(2\lambda \right)} is a simplified version of the equation of time .
The December-solstice solar year is the solar year based on the December solstice. It is thus the length of time between adjacent December solstices. The length of the December-solstice year has been relatively stable between 6000 BC and AD 2000, in the range of 49 minutes 30 seconds to 50 minutes in excess of 365 days 5 hours.
As at Feb 20, 2025, solar cycle 25 is averaging 37% more spots per day than solar cycle 24 at the same point in the cycle (Feb 20, 2014). Year 1 of SC25 (Dec 2019 to Nov 2020) averaged 101% more spots per day than year 1 of SC24. Year 2 of SC25 (Dec 2020 to Nov 2021) averaged 7% more spots per day than year 2 of SC24.
The solstice usually – but not always – takes place on December 21. The date that the solstice occurs can move forward or back by a day because the solar year (the time it takes for the sun to ...
On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). More simply, 1→2 is a complete ...
The Earth's axial tilt changes slowly over thousands of years but its current value of about ε = 23.44° is nearly constant, so the change in solar declination during one year is nearly the same as during the next year. At the solstices, the angle between the rays of the Sun and the plane of the Earth's equator reaches its maximum value of 23.44
The Islamic calendar is a purely lunar calendar and has a year, whose start drifts through the seasons and so is not a solar calendar. The Maya Tzolkin calendar, which follows a 260-day cycle, has no year, therefore it is not a solar calendar. Also, any calendar synchronized only to the synodic period of Venus would not be solar.