Drawing Options - Grid
When the Grid checkbox is selected, cross-hairs appear on the image in the main window.
The scale of the image is also indicated in the upper left of the image. When you use the Grid function with zoom, you see the scale for the image change.
For more information about the units used to measure distance across the sky, return to Pre-flight Training, Sky Orientation.
The distance between each mark along the edge of the image is 2 arcminutes. One arcminute is approximately the amount of sky covered by the thickness of 2 playing cards held at arm's length.
The scale on this zoomed-in galaxy image is 10 arcseconds between each white mark. Ten arcseconds in approximately equal to the amount of sky covered by the width of a human hair held at arms length.