Like the Earth, the planets in the solar system rotate on their axes as they revolve around the Sun.

This means that if you shoot a video using lucky imaging, there is a limit to how long you can shoot before detail will be smeared by the rotation of the planet in the final composite image.

As we look at a planet in the sky with the unaided eye, most appear to rotate from celestial east to celestial west. Details will appear first on the eastern limb, move towards the central meridian, and disappear around the western limb.

How long you can shoot before planetary rotation is a problem is determined by four basic factors.

1. Rotation speed - You will be able to shoot the planets that rotate the fastest for a shorter period of time before details begin to blur.

   Imagine shooting a video of Jupiter which takes 9 hours 55 minutes and 30 seconds to rotate once on its axis. If you shot a video for that long, you would get an entire rotation of Jupiter where you could see all of the surface detail. But when you ran that video through RegiStax, AviStack or AutoStakkert! to pick out only the sharpest frames and stack them, you would end up with a jumbled blur of different features from different parts of the planet as it rotated.

2. Detail Location - Details will usually move the fastest when they are on the planet's equator when they cross the central meridian facing us.

   Details closer to the poles will move slower. Details just coming around the eastern limb will not smear as much because they are heading more towards us than laterally across our field of view. Likewise details going away from us that will disappear eventually behind the planet on the western limb will not smear as much.

   The length of our videos will be limited here by details that are moving the fastest near the planet's equator and central meridian.

3. Planetary distance from the Earth - An object that rotates at a given speed can be shot for a longer time before details blur if it is farther away.

   Jupiter and Saturn, for example, have similar rotation periods, but Saturn is nearly twice as far away. That means the same sized detail on Saturn will be only 1/2 the angular size, and that we should be able to shoot it for twice as long before it blurs.

4. Focal Length - More focal length produces more magnification which gives us a larger image scale. But this also increases the blurring effect of planetary rotation.

   The longer the focal length you use, the shorter the length of video you can shoot.

The length of time that we will be able to shoot video for will be mostly determined by the planet's rotation speed and focal length.

Detail location on the planet is limited by the fastest moving detail on the equator and meridian. This is where we will be able to best resolve it as it is not foreshortened as it is when it is closer to the limb.

Planetary distance is relatively the same for a given planet in the outer solar system, but can be important for planets in the inner solar system. But usually we will be trying to shoot both inner and outer planets when they are closest to us so that they have the largest apparent size.

Venus shows no surface detail because it is shrouded in clouds. Some detail can be seen in the clouds if Venus is shot in ultra-violet light. Unfortunately stock DSLR cameras are not very sensitive to ultra-violet. They would need to be specially modified to be used for UV photography of Venus.

Venus's clouds rotate at a much faster rate than the planet itself. Since the surface is not visible because of the clouds, both figures are given in the chart below. For ultra-violet photography of the clouds of Venus, their period of rotation should be used instead of that of the planet.

Mars can vary quite a bit in size depending on where it is in its orbit in relation to the Earth. When it is smaller, we can shoot longer videos.

One nice thing about shooting the planets with a DSLR is that they are one-shot color cameras. We don't have to deal with individual red, green and blue filters to create color like with a monochrome camera, or worry about dividing our limited video time into three for these filters.

RegiStax and WinJUPOS have features that will attempt to correct for movement of planetary features due to rotation.

Maximum Smear Allowed

In the calculator below, we have left out distance as a specific criterion and based the calculation on how much smear we are willing to allow in terms of arcseconds because this incorporates distance.

The question now becomes how much smear is tolerable. This will, of course, depend on the quality of the seeing and aperture of the telescope in terms of how much fine detail is present and how much could theoretically be recorded.

For example, if we are shooting with a small 4-inch refractor under poor seeing, we can significantly relax our maximum smear allowed. If we are shooting with a 16 inch telescope with excellent optics under excellent seeing, we will need to make this factor more strict.

One-half arcsecond of detail smearing seems like a reasonable place to start for a 6-8 inches of aperture under good seeing conditions.

Referencing the table below we can see that Jupiter has the fastest rotation period at about 9 hours and 55 minutes. To capture Jupiter's very fine detail, we can't shoot video for too long before it will smear.

Saturn has a similar rotation period of 10 hours and 14 minutes. Saturn, however, does not generally have the same amount of high-resolution detail on the ball of the planet, unless there are details from any storms that may be going on. If there aren't any storms, and you don't have the seeing or large scope with great optics to try to capture some incredibly fine detail in the rings like the spokes, then you can record video of Saturn for a longer period of time.

^* A solar latitude of 26° is the latitude of most sunspot activity.

Minus numbers indicate that the planet has retrograde rotation opposite most of the other planets in the solar system. Enter these planet's rotation periods as a positive number in the calculator below however.

Maximum Exposure Length for Planetary Rotation

Example: If Jupiter subtends 47 arcseconds, and has a rotation period of 595.5 minutes, and we are willing to allow 0.5 arcseconds of detail smearing due to Jupiter's rotation, how long can we record a video?

Pressing the "Calculate" button shows we can shoot Jupiter for about 2 minutes before rotation begins to smear details.

Simply highlight the numbers in the appropriate boxes and delete them and enter your own figures based on the table of planetary sizes and rotation periods given above. Do not use commas for large numbers. For planets whose sizes vary a lot, look up the size of the planet at the time you want to shoot it in a planetarium program and enter that information.