Visibility Measures

Since Astronomical objects come in various shapes, sizes, energies and distances from us, there is a lot of variance in their ease of observation.

Some are bright enough to see with the naked eyes, such as

  • the Sun and the Moon
  • nearby planets in our solar system including Mercury, Venus, Mars, Jupiter and Saturn
  • thousands of bright stars in our stellar neighbourhood
  • the Andromeda Galaxy
  • the Orion Nebula
  • the spiral arms in our galaxy, the Milky Way

Other objects are within the reach of amateur astronomers but require aides such as a camera, a telescope or a pair of binoculars.

Knowing the visiblity measures of the objects as well as the capabilities of your observing equipment, will help you plan your observation sessions.

For the purposes of Amateur Astronomy, two visibility measures are typically used:

  • Apparent Magnitude
  • Surface Brightness

Each measure is suitable to compare different types of objects, and SkEye uses the most suitable measure automatically for a given object.

Apparent Magnitude

Apparent Magnitude is a measure of the brightness of a star or other astronomical object observed from the Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's light caused by interstellar dust along the line of sight to the observer.

This visibility measure is best suited for point sources of light, such as stars and open clusters.

The magnitude scale is reverse logarithmic: the brighter an object is, the lower its magnitude.


ObjectApparent Magnitude as seen from Earth
Full Moon−12.90

For extended and fuzzy objects, such as galaxies and nebulae, the Apparent Magnitude is calculated as the integrated magnitude for the angular area subtended by the object. It is not directly useful as a visibility measure for such objects since information about the size of the object is lost.

Surface Brightness

Surface Brightness is defined as the Apparent Magnitude per unit angular area of an object.

Since the area is accounted for, it is suitable for comparing visibility of extended and fuzzy objects.


A truly dark sky has a surface brightness of 21.8 Mag/arcsec2. The peak surface brightness of the central region of the Orion Nebula is about 17 Mag/arcsec2.