This tool measures your own photograph of the Sun in kilometres: how high a prominence stands above the limb, and how long a filament runs across the disc. Open the photo and it finds the disc by measuring its limb. The Sun’s radius is 695,700 km, so the disc’s radius in pixels sets the scale, whatever telescope or camera took the picture. Click to measure. Rings every 50,000 km above the limb, with Earth and Jupiter drawn at the same scale, let the picture be read at a glance, and the saved image keeps them with your measurements.
How to use it
- Open a photo of the Sun: a JPEG, PNG or TIFF. Prominences and filaments show in H-alpha and calcium K; sunspots and their groups in white light. Nothing is uploaded — the file is read in your browser and goes no further.
- Check the disc. The tool measures the limb, fits a circle to it, and says how closely it fitted and what one pixel is worth in kilometres. Where cloud or a cropped edge defeats it, open Advanced and type the centre and the radius in pixels.
- Click off the disc, at the top of a prominence. That measures its height, straight out from the limb beneath it, and it is read out live as you move before you click. To place a point more closely, zoom in with the mouse wheel, a pinch or the + button, up to 16 times, and drag to move around.
- Click on the disc to start a length, then along a filament, point by point, following its curve. Click the last point again, or press Enter, to finish. Undo, or Backspace, takes the last point off. A click off the disc finishes the length too, and measures a height there.
- For the width of a prominence, or anything measured straight, choose Straight line above the view. Clicks then draw a straight line anywhere, on the disc or off it, measured across the sky rather than along the surface. Choose Auto to go back.
- Read the measurements down the rail, in kilometres and in Earth diameters. Each is labelled on the photo — H1, H2 for heights, L1, L2 for lengths — and can be removed on its own.
- Turn the rings off where they hide something, and save a PNG at the photo’s own resolution with the rings, Earth, Jupiter and the measurements on it.
How it works
The scale comes from the Sun itself. Its radius is 695,700 km, the IAU’s nominal value, so a disc 680 pixels in radius puts 1,023 km in each pixel at its centre. No focal length, pixel size or date is needed, and a cropped photo works as long as enough of the limb shows to fit a circle to. In H-alpha the edge the tool measures is the chromosphere’s, a few thousand kilometres above the photosphere: under 1% of the radius.
A height is measured off the disc, along the line from the centre through the point you click, from the limb to that point. It is the height in the plane of the sky, and so a lower bound: a prominence leaning toward or away from Earth stands taller than it measures, and one a little beyond the limb has its foot hidden behind it.
A length is measured along the surface. The disc is a sphere seen flat, so each point you click on it is lifted back onto the sphere, and each pair of points is joined by the shortest path between them over the Sun, a great circle. The line drawn follows that path and bends as the surface does. Near the limb the surface is seen edge on: a stretch that looks short is long, and an error of one pixel grows, more than threefold past 95% of the radius, which the tool flags.
The rings stand 50,000, 100,000, 150,000 and 200,000 km above the limb. Earth, 12,756 km across, and Jupiter, 142,984 km, are drawn in the corner at the same scale, so a prominence can be told in planets as well as in kilometres.
Formula
h = (d ÷ r − 1) × 695,700 km
h is the height above the limb. d is how far the clicked point is from the centre of the disc, and r is the disc’s radius, both in pixels. 695,700 km is the Sun’s radius.
Worked example
On a disc 680 pixels in radius, the top of a prominence clicked 742 pixels from the centre gives h = (742 ÷ 680 − 1) × 695,700 = 63,400 km to the nearest hundred: about 5.0 Earth diameters, between the first and second rings.
Common questions
Why is the height of a prominence a lower bound?
Because the photo shows it flattened onto the sky. A prominence standing straight up from the limb measures its full height, but one leaning toward you or away from you is foreshortened, and one rooted a little behind the limb has its base hidden. Each makes it measure shorter than it is, never taller.
Why is a filament near the limb longer than it looks?
The Sun is a sphere, and near the edge of the disc its surface is seen almost edge on, so distances along it are squeezed together on the photo. The tool measures along the surface rather than across the picture, which undoes the squeeze. The same squeeze makes a pixel’s error count for more there, which is why lengths close to the limb are flagged.
Do I need to enter my focal length or pixel size?
No. The Sun’s radius is known, so the disc’s own radius in pixels is the scale. That holds for any telescope, camera, Barlow or crop, and for a mosaic too, as long as the disc is round and enough of the limb shows.
How accurate are the measurements?
To about a pixel at each end, which on a full-disc photo is a thousand kilometres or so; the rail says what a pixel is worth on yours. That is why the results are rounded to the nearest hundred kilometres. A prominence’s faint top and a filament’s fuzzy ends are usually a larger uncertainty than the clicks.
Which photos can I measure?
Any photo of the Sun with a round, sharp limb: a full disc or a crop, in H-alpha, calcium K or white light. Processing does not matter, as long as the image has not been stretched out of round. An inverted or colourised image works the same.
Is it safe to photograph the Sun?
Only through a proper solar filter fitted over the front of the telescope, or a dedicated H-alpha or calcium K solar telescope, checked before every use. Never look at or photograph the Sun without one. This tool reads a photo you already have.
Related tools
In the store
Equipment this tool helps you choose, at David Astro: