Date Calculator
Add or subtract years, months, weeks and days, or find the exact span between two dates, with the day of the week.
CyclesSynodic-Period Calculator
How often two bodies line up, the average time between alignments, plus the pairwise periods for three or more.
CyclesCycle Length Calculator
Enter any span of time in any unit and see it in every other unit and as a multiple of each astronomical cycle, from the lunar month to the Great Year, with the nearest cycles surfaced for you.
AlignmentsPlanetary Alignment Calculator
The real dates of planetary conjunctions and oppositions, and how often three or more planets line up.
ReturnsCelestial Return Calculator
Where the Sun, Moon and planets sat on any date, and when each one returns to that spot: the solar, Jupiter and Saturn returns.
OpticsTelescope & Eyepiece Calculator
Magnification, exit pupil, true field of view and useful-magnification limits for every eyepiece, plus what your sky and aperture can realistically show tonight and whether a target fits your camera's frame.
SunSunrise, Sunset & Dark Window
Sunrise, sunset, solar noon, day length and the three twilights, plus the moonless dark window and the next 30 nights ranked for observing.
NumbersPythagorean Numerology
Add up the classic Pythagorean letter values for words and names, with a running total across them all.
About these tools
Every calculator here runs entirely in your browser. Nothing is sent to a server, there is no signup, and each one shows the method behind its result so you can check the working. The Synodic-Period Calculator uses a built-in astronomy engine to compute real conjunction and opposition dates, not just average periods.
Looking for the reference list or the lessons? Browse every astronomical rhythm in the cycles by length guide, or explore the interactive explainers in the Learn section.