Calculators · The workbench
Bring what you are already holding.
Fourteen calculators that turn it into what you want to know: a date into a weekday, two planets into the date they next meet, an aperture and an eyepiece into what you will actually see through them, a camera lens into how long the shutter can stay open. Every one of them shows its working.
No signup, no key and no account, and nothing you type leaves your device. Where two accepted formulas disagree, both are printed rather than one quietly chosen for you. How this is computed →
- Julian dateThe continuous day count
- SiderealGreenwich apparent
- The SunApparent place, of date
- Your meridianLocal sidereal time
The last row needs a longitude.
- 14calculators, every one free
- $0no signup, no key, no account
- 1700 to 2200the years the engine computes
- 0of what you type leaves your device
Start here: plan tonight
Three of these get opened within minutes of each other, because they answer the three questions that decide an observing session: when it gets properly dark, what the instrument will show, and how long the shutter can stay open.
Every calculator, by what you bring
The whole workbench, including the three above, indexed by the thing you are holding when you arrive. The left column is the input; the question on the right is the one that page exists to answer.
Plan tonight's session
Everything you settle before walking outside: when it gets properly dark where you are, what the instrument will actually show, and how long the shutter can stay open. These are the three above, in their place in the index.
- your place Sunrise, Sunset and Dark WindowWhen does it get properly dark tonight, and which of the next thirty nights is the most moonless?
- your scope Telescope and EyepieceWith this aperture and that eyepiece, what magnification and true field do I get, and will the target be more than a smudge?
- your camera Astrophotography ExposureHow many seconds can I hold the shutter open before the stars stop being points?
Date a sky event
All three take bodies, plus a date or a range, and hand back dates. They run on the same ephemerides, so a conjunction found in one is the conjunction the others agree on.
- two bodies Synodic PeriodHow long is the wait between one Jupiter-Saturn conjunction and the next, and when exactly was the last one?
- two bodies and a range Planetary AlignmentOn exactly which dates do Mars and Jupiter meet, and when do three or more planets really fall into line?
- a date Celestial ReturnWhere was Saturn in the zodiac on the day I was born, and when does it next come back to that spot?
Read the sky as an instrument
Both turn an instant and a longitude into an hour angle. Local sidereal time and the almanac's Aries hour angle are the same quantity from the same call, which is why the two pages agree to the second.
- a longitude Local Sidereal Time ClockWhat right ascension is crossing my meridian right now, so I know what is highest tonight?
- a sextant sight Nautical AlmanacI shot the Sun's lower limb at 45 degrees. Where does that line of position put me?
Reckon time and orbits
Numbers in, numbers out: no location, no ephemeris, no clock. These three work exactly the same indoors, offline, or on the far side of the planet.
- a date, or two Date CalculatorIf I add three months and ten days to this date, what do I get, and what weekday is it?
- a span of time Cycle Length CalculatorHow many lunar months and Saros returns fit in this span, and is there a real rhythm that length?
- an orbit Kepler's Third LawHow many minutes does one lap take at 420 km up, and how high must an orbit be to take a full day?
Put yourself in the picture
The input here is personal and the intent is curiosity rather than observing. They are honest about the physics anyway: the weight page rewrites its own headline in kilogram mode, because mass does not change when you land somewhere else.
- a birth date Age on Other PlanetsHow many Mars years old am I, and what is the calendar date of my next Mercury birthday?
- your weight Weight on Other PlanetsWhat would the scale actually read on Mars, and does my weight or my mass change when I get there?
- a name Pythagorean NumerologyWhat do the classic Pythagorean letter values add up to, and how does that differ from the Chaldean count?
New to sight reduction? Start with the interactive explainer on how celestial navigation works. Looking for the rhythms rather than the arithmetic? Browse every astronomical rhythm in the catalog by length, or read the concepts explained.
How these compute
Every calculator on this page runs entirely in your browser. Nothing you type is sent to a server, there is no signup and no key, and each one shows the working rather than just the answer, so you can check it or argue with it. The ones that need real positions call the same vendored astronomy engine as the rest of the site, which is why the Synodic Period Calculator returns the actual dates of the last and next conjunction instead of dividing a pair of average periods.
Where the field has more than one accepted convention, both are printed. The telescope page gives the Dawes limit and the Rayleigh criterion together; the exposure page gives the 500 rule and the NPF rule together and then the trail length that settles the argument; the almanac carries its own educational-use disclaimer rather than implying it is a certified publication. The conventions behind all of it are on the methodology page.
A calculator missing from this bench, or a result worth arguing about? Write to info@cyclecalcs.com.