The Jupiter Great Red Spot Calculator tells you when the Great Red Spot will cross Jupiter’s Central Meridian, the moment the storm faces Earth and is easiest to see. It lists the next 5, 10 or 20 of these Transits in your own time zone, each with a viewing window, and it asks for neither a location nor an account.
How to use it
- Under Start from, leave Now selected to see what is coming up. To plan another night, choose Pick a time and enter the date and time you want. Times are read in your own time zone.
- Under Transits to show, choose 5, 10 or 20. Each row of the list is one Transit, with its time, its date, its “Best window” and its “Time away”.
- The Next transit panel gives you the coming Transit’s time, date and window, along with a countdown such as “in 3h 57m”. The list refreshes itself every 30 seconds.
- Leave “GRS longitude” on Estimate to use the tool’s drift model. If you know the spot’s current System II longitude, switch to Manual and type it in degrees. The field shows the estimate as a hint.
- The readouts at the top right give you Central Meridian now, which is the System II longitude facing Earth in degrees, and System II rotation, the mean time between the Transits in the list.
- The picture of Jupiter shows the planet the way the naked eye or binoculars do, with north up and east on the left. The Great Red Spot is drawn where it is now, and the Central Meridian runs dashed down the middle. The spot comes in at the left limb and crosses to the right, and the line above the picture tells you how many degrees it is from the meridian, or that it is round the far side. Drag the slider underneath to run the picture ahead through one rotation and watch the spot come round. The Transit mark shows when it crosses.
- Finally, check how high Jupiter stands from your Location, because the list does not. A Transit that happens with Jupiter below your horizon, or in daylight, cannot be seen.
How it works
Jupiter turns once in about 9 h 55 m, so the Great Red Spot comes back to the Central Meridian, the line from Jupiter’s centre to Earth, about every 9 h 56 m. The tool works in System II longitude, the rotation frame used for Jupiter’s mid-latitudes. The Central Meridian sweeps through that frame at 870.19° a day, or 36.3° an hour, and a Transit is the moment its longitude equals the spot’s. To find that moment, the tool first estimates how far the sweep still has to run, then repeats the calculation with the full model until the two longitudes agree to within 0.00001°.
The Central Meridian’s longitude comes from Project Pluto’s closed-form fit, which is good to about a minute of time. Since a Transit happens at the same instant everywhere on Earth, the tool never needs to know where you are. It shows the time on your device’s clock and names the time zone above the list.
The weak point is the spot’s own longitude, because the spot drifts. The tool models that drift as a straight line, anchored at 91.0° on 1 June 2026 and moving 1.75° a month towards the west. Every degree of error shifts a Transit by about 1.7 minutes, and the tool warns you once the anchor is more than 1.5 years old. For a particular night, you can choose Manual and enter a longitude read from JUPOS, and the tool will hold it fixed for every Transit in the list.
The “Best window” runs 50 minutes either side of each Transit, which is about 30° of rotation. The spot actually faces Earth for about 5 hours in all, but it looks foreshortened near the edge of the disc, so the window marks the stretch when it is well placed.
Formula
wait = ((λspot − λCM) mod 360°) ÷ 870.19 °/day
λspot is the Great Red Spot’s System II longitude and λCM is the Central Meridian’s, both in degrees. Taking mod 360° wraps the difference into the range 0° to 360°. 870.19 °/day is the rate at which the Central Meridian sweeps round. The wait comes out in days, so multiply it by 24 to get hours.
Worked example
At 06:00 UT on 1 June 2026, the tool puts the Great Red Spot at 91.0° and the Central Meridian at 307.6°. The difference is 91.0 − 307.6 = −216.6°, which wraps to 143.4°. Dividing gives 143.4 ÷ 870.19 = 0.1648 days, or 3.955 h, which is 3 h 57 min. That puts the Transit at 09:57 UT. The tool then refines this estimate and gives 09:57:19 UT.
Common questions
When is the Great Red Spot facing Earth tonight?
Open the tool with Start from set to Now. The Next transit panel gives you the time in your time zone with a countdown, and the list shows the Transits that follow. Whether you can actually see the spot depends on Jupiter being above your horizon and the sky being dark at that time, and the list checks neither.
How often does the Great Red Spot cross the Central Meridian?
About every 9 h 56 m, since Jupiter turns once in about 9 h 55 m. The System II rotation readout gives the mean spacing of the Transits in the list. Because that spacing is not 24 hours, the Transits fall at a different clock time each day, and only some of them happen at night.
How accurate are the Great Red Spot transit times?
The Central Meridian is good to about a minute of time. Most of the uncertainty comes from the spot’s own longitude, which drifts and is estimated from a model. Each degree of error moves a Transit by about 1.7 minutes, so an estimate that is 3° off puts the time out by about 5 minutes. For a particular night, enter a longitude measured on JUPOS.
How do I enter the current longitude of the Great Red Spot?
Look up its System II longitude on JUPOS, switch the longitude setting from Estimate to Manual, and type in the degrees. The tool keeps that value fixed for the whole list. While the entry is blank or only half typed, the estimate stays in use.
Does the Jupiter Great Red Spot Calculator need my location or an account?
No. There is no account, and the tool never asks where you are. The only things kept are two preferences in your browser: your theme and your language.
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