Lunar Labels

Lunar Labels names the craters, maria, mountains and other features on your image of the Moon. BiggSky finds where the Moon is in the image and how it is turned, then draws the official names (the IAU Gazetteer of Planetary Nomenclature, about 9,000 features) on the View. The labels follow the zoom: zoom in and smaller features appear, and the text stays the same size on screen.

Lunar Labels works on any lunar image shown in the View: a frame, a stack, a deconvolution or a mosaic. It does not change the image.

Quick start

1. Show your lunar image in the View.
2. If the image has no capture time, set the Date box to the night it was taken (see Where the time comes from below).
3. Press Calc. BiggSky places the Moon (about 20 seconds) and ticks Labels.
4. Zoom and pan as usual. Untick Labels to hide the names; untick Outlines to hide the crater outlines.

If Calc cannot place the Moon (a close-up with no edge of the Moon in it, or part of the disk with no date), it offers Mark Features: see Mark below.

The Lunar Labels panel

Placing the Moon with Calc

Calc finds the edge of the Moon, then matches the image to a map of the whole Moon, lit as it was at the time the image was taken. It works out where the center is, the size of the disk, how the image is turned and whether it is mirrored (by a diagonal, for example), and the tilt of the Moon toward us on that date (its libration). The result is accurate to a few pixels across the whole disk.

Calc needs part of the Moon's edge in the image, and either the capture time or most of the disk (70% or more). Without a time, BiggSky looks through the last three years for a Moon of the same phase that matches; this takes about 45 seconds instead of 20.

What is lit is taken from the image itself, so features in the dark part of the Moon are not labeled, whatever the time.

#### Where the time comes from

BiggSky uses the first of these that it finds:

1. the frame times or the header of the SER video the image came from;
2. a name in the WinJUPOS style, such as 2026-09-26-2112_9 (UTC);
3. the Date box;
4. a date in the dataset's names or folders, such as Moon_2026-09-26.

A date alone is enough: BiggSky searches that night and a day either side, in any time zone. The Date box is used by every image you Calc until you clear it.

Mark: place the Moon from features you know

Mark places the Moon from features you click on the image. Use it when Calc cannot:

• a close-up that does not show the edge of the Moon;
• part of the disk (one panel of a mosaic, say) when you do not know the date;
• an image Calc placed wrongly.

Mark needs no edge of the Moon and no date.

1. Press Mark. The Mark lunar features window opens beside BiggSky.
2. Type part of a feature's name, for example coper, and choose Copernicus from the list.
3. Click the center of Copernicus on the image. A cyan mark with its name appears there. Drag the mark to move it, or choose the name again to place it again. Press Esc instead of clicking to cancel.
4. Mark more features the same way. Two are enough when much of the lit Moon is in the image. Mark three or more if the image may be mirrored, and four or more for BiggSky to work out the date as well.
5. Press Place the Moon. The labels appear, and the table shows how far each mark is from where the others put its feature (Off (px)).

#### Snap to the craters in the image

A mark clicked by eye is often 10 to 30 pixels from the true center of its crater on a close-up. With Snap to the craters in the image ticked (the default), BiggSky does not stop at your marks. Once they have placed the Moon roughly, it looks for each lit crater near where the labels put it, by its rim: the rim's shadowed side and its sunlit side. It then places the Moon on the craters it found, often 30 to 60 of them, and does this again until the placing stops changing. It takes a few seconds.

• Your marks on craters it found move onto those craters. The status line says how many craters were used and how many of your marks moved.
• The Off (px) column then shows how far your other marks are from their features.
• Two well-chosen craters are often enough to start. On a close-up, the labels then land within a few pixels across the image, where the marks alone left them 20 to 70 pixels off.

Snapping needs craters with shadows. It works best in a low Sun, near the terminator, and finds little on a full Moon. If it cannot match enough craters, or the craters it finds would move the Moon away from your own marks, it says so and the Moon is placed from your marks alone. Untick the box to place the Moon from your marks only.

Choose features you are sure of, spread across the image. Large, sharp craters marked at the middle of their floor work best: Copernicus, Tycho, Plato, Theophilus, Petavius, Clavius. Features close together, or all along one edge, place the Moon less well.

Read the Off column. A few pixels is normal. One mark far off the others (BiggSky says which) is usually a wrong name or a mark on the wrong crater: check it, move it, or select its row and press Remove. A mark can also be deleted by right-clicking it and choosing Delete Point.

The two boxes at the bottom of the window are optional:

• Pixel scale, arcsec/px. If you know your image scale, enter it. It helps most with two marks on a close-up. Leave it at 0 if you do not know it.
• Year taken, if no date. With four or more marks and no date, BiggSky estimates the date from where the marks put the Moon and from the lighting. The Moon's tilt and phase nearly repeat over the years, so without a year several dates can fit equally well; the year usually settles it.

#### What Place the Moon fills in

After Place the Moon, BiggSky fills in what it found:

• Pixel scale. If the box was empty, it is filled with the scale of your image, from the size of the disk. With a date it is accurate to about half a percent. Without one it is only good to about 6%, because the Moon's distance, and so its size, changes through the month.
• The date. When one date fits clearly better than any other, BiggSky shows it, puts its year in the Year box and, if the Date box is empty, sets the Date box to it. The next images of that night can then be placed with Calc. When several dates fit about as well, it lists them, best first, and fills in nothing. The labels do not need the date.

The marks are kept with the image. Open Mark again to see them, move them, or add more and press Place the Moon again.

Pressing Calc on an image you placed with Mark replaces your placing only if Calc succeeds; if it fails, the Moon stays placed from your marks.

Mosaic panels and frames

Each frame keeps its own placing. When a dataset holds several images of the same size (the panels of a mosaic, or the frames of a video), each is placed on its own: show panel 2 and press Calc, show panel 1 and press Calc (or Mark), and each shows its own labels when you go back to it. A frame that has not been placed shows no labels until you Calc or Mark it; it never shows another panel's labels.

A new render, stretch or white balance of the same frame keeps its labels.

Reset

Reset forgets how the Moon was placed in the frame shown, here and in the project, removes its labels, and closes the Mark window. The other frames keep theirs. Press Calc or Mark to place the Moon again.

You rarely need it: each frame keeps its own placing (above). Use Reset when the labels on a frame are wrong, for example after Calc placed a frame with the wrong date, or when you want to start the marks of a frame again.

Saved with the project

How the Moon was placed in each frame is saved with your project. When you open the project again, the labels come back without a new Calc: tick Labels. BiggSky first checks that the saved placing still fits the image shown, so an image that was flipped, rotated or cropped since is not labeled from it: press Calc again for that one.

Pressing Calc on an image whose Moon is placed already asks first, and says how and for which date it was placed. Show Labels shows the labels from that placing at once; Calc Again analyzes the image once more, which takes about 20 seconds. Use Calc Again (or Reset) when the labels are in the wrong place.

Find

Find opens a search window: type part of a name and choose a feature. BiggSky draws its outline and name in yellow and moves the view to it, zoomed so the feature fills a good part of the window. Each feature you find stays marked until you press Clear. Find needs the Moon placed in the image (Calc or Mark; the Find window has its own Mark… button). Find is off while the quiz is on.

Lunar Quiz

Tick Take the quiz to test yourself on the image shown. BiggSky outlines a feature and offers three names; choose one and press Submit. Hint first tells you the kind of feature, its size and where it is; press it again (50/50) to remove one wrong answer. A right answer after a hint scores half a point. Choose the Level:

• Beginner: the maria and about 80 of the best-known craters and features.
• Intermediate: craters 40 km and larger; lakes, bays and marshes; mountains, scarps and capes 100 km and larger; rilles, ridges, crater chains and valleys 150 km and larger.
• Expert: everything else, with wrong answers chosen from nearby features of the same kind.

The questions come from the whole lit Moon in the image, not only the part you have zoomed into. The quiz needs the Moon placed (Calc or Mark).

Lunar Movie

A 50 or 100 megapixel mosaic shows little of its detail on a screen. Lunar Movie makes a video tour of it: from the whole Moon it flies to the places you choose, holds on each with its labels and a caption, and moves on, with a fade from black at the start and to black at the end.

1. Zoom and pan the View to the first place, for example the whole Moon, and press Add Key. A row appears in the table, named after the feature the view is centered on.
2. Do the same for each place, in the order the movie visits them.
3. Choose the Size and press Save Movie. BiggSky asks where to save the MP4 and makes it, showing its progress; Cancel stops it.
4. Press Play to watch it in your video player.

For a quick movie, press Auto instead of steps 1 and 2. It makes a ready tour: the whole image, then up to six well-known craters and features that are in it and lit (Tycho, Copernicus, Plato, Aristarchus and the like, spread across the image, in the shortest round), then the whole image again without labels. Auto needs the Moon placed (Calc or Mark). Edit the table afterward as you like: remove a feature, change a hold, or Add your own.

The sizes offered are the LunarMovieSizes setting (Settings tab, Lunar Movie): add your own as width x height, separated by commas, for example 2560 x 1440, 1080 x 1350.

BiggSky decides how to move between two places: a place close to the last one is panned to, and a farther one is flown to, zooming out until both are in view and then in again.

In the table:

• Key is the caption. A feature's name gives its outline and a caption with its kind, size and origin; type any other text for a caption of your own, or clear it for none.
• Hold (s) is how long the movie stays there.
• Labels shows the labels and the caption during the hold, as the Lunar Labels panel draws them (font size, density and outlines), with fewer names than on screen.
• Double-click a row to show its place in the View. Right-click for Move Up, Move Down, Remove and Remove All.

The watermark shows at the lower right at the start and the end of the movie:

• Watermark ticked shows the BiggSky logo, or your own image: press Browse to choose one (a PNG with a transparent background looks best). BiggSky remembers it; press Browse again and choose BiggSky Logo to go back.
• Text adds your words, such as your name, beside the image, or alone when Watermark is unticked.

The movie follows the image as the View shows it, with its stretch and white balance, and turned as it is. It zooms in to 1.5 screen pixels per image pixel at most. The labels need the Moon placed (Calc or Mark); without that the movie has no labels. The key views are saved with the project.

Tips and troubleshooting

• "No lunar limb was found." The image does not show the Moon's edge. Use Mark.
• "The capture time is not known and less than 70% of the disk is in the image." Set the Date box and press Calc again, or use Mark.
• "The Moon could not be placed with confidence." The image is too soft, too small, or shows too little of the lit Moon for Calc. Use Mark.
• "The marks do not agree: … is … px from where the others put it." Check that feature's name and place; move or remove its mark.
• "The image may be mirrored." Two marks cannot tell a mirrored image from a normal one when the image itself does not settle it. Mark a third feature, away from the line through the first two.
• Labels in the wrong place on one frame. Press Reset on that frame, then Calc or Mark it.
• No labels after switching panels. That panel has not been placed yet: Calc or Mark it.

Feature names: IAU Gazetteer of Planetary Nomenclature (USGS). See also the Mosaic manual and the full BiggSky manual.