Astronomy Definitions

Magnitude

Magnitude which comes from the term “apparent magnitude.” Astronomers use this term to describe the brightness of an object in the night sky. Apparent magnitude originally used a scale from 1 to 6, with 1 representing the magnitude of a particular reference star and 6 representing the faintest object that can be seen by the naked eye.

Each level represents a change in brightness of 2.5 times. So an object of magnitude 3 is 2.5 times brighter than an object of magnitude 4. The important point to remember is that brighter objects have smaller magnitudes and fainter objects have larger magnitudes. Very bright objects have negative numbers. For example, the brightest star in the sky is Sirius with an apparent magnitude of -1.6. Sirius is near the constellation Orion in the winter sky.

Some examples of objects with varying magnitudes are listed below.

Apparent MagnitudeCelestial Object
-26.7 Sun
-12.6 Full Moon
-4.4 Venus (at brightest)
-3.0 Mars (at brightest)
-1.6 Sirius (brightest star)
+3.0 Naked eye limit in an urban neighborhood
+5.5 Uranus (at brightest)
+6 Naked eye limit
+9.5 Faintest objects visible with binoculars

NELM

Naked Eye Limiting Magnitude or NELM represents the faintest magnitude that your naked eye can see in the conditions being measured. A NELM of 6.0 means you will be able to see objects as faint as magnitude 6.0; this is about as faint as the human eye can see. On the other hand, a NELM of 2.0 means the sky is relatively bright. This could be due to the moon or your location, say in an urban area with lots of light pollution. 

Some of the darkest locations on Earth have a NELM of around 7.0 to 7.5 - very dark, indeed.

Mag/Sq Arcsec

Astronomers also use Mag/Sq Arcsec to measure relative sky darkness. Mag/Sq Arcsec is the apparent magnitude for a certain square area of the sky measured in arc seconds. 

A square arc second is a square area of the sky that has one arc second on each side. An arc second is a very small measurement. For example, Jupiter looks very small in the sky, but through binoculars it does have a disk-like shape and a small diameter. Jupiter’s angular diameter ranges from 30 to 50 arc seconds. 

Therefore, Mag/Sq Arcsec means the brightness (expressed in apparent magnitude) of a one square arc second area of the sky.