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.
OrbitsKepler's Third Law Calculator
Orbital period from distance and back, around the Sun, another star, or Earth: from the ISS's 93 minutes to Pluto's 248 years.
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.
TimeAge on Other Planets
Your age in every planet's years and days, and the date of your next birthday on Mars, Jupiter and the rest.
GravityWeight on Other Planets
What you would weigh on the Moon, Mars, Jupiter and the Sun, with the honest physics of mass, gravity and cloud tops.
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.
ImagingAstrophotography Exposure
How long you can expose before stars trail: the 500 rule and the NPF rule side by side, with the trail length your camera would record.
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.
TimeLocal Sidereal Time Clock
The sky's own clock, live: Greenwich and local sidereal time, and the right ascension crossing your meridian right now.
NavigationNautical Almanac
Live GHA and declination for the Sun, Moon and planets, plus a sextant sight-reduction calculator.
NumbersPythagorean Numerology
Add up the classic Pythagorean letter values for words and names, with a running total across them all.
About these calculators
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.
New to sight reduction? Start with the interactive explainer on how celestial navigation works. Looking for the reference pages or the lessons? Browse every astronomical rhythm in the cycles by length guide, or read the concepts explained.