Orpington Astronomical Society
Brightness, Size, Distance, and What You Can See

Brightness, Size, Distance, and What You Can See

Sense of scale, scope and movement

If you have dark skies, you’ve probably seen stars that are thousands of light-years away. Others are less than 10 light-years away. A light-year is the distance light travels in one year, moving at 186,000 miles per second.

Our Milky Way Galaxy is more than 100,000 light-years across. It contains billions of stars, but the most adept skywatchers can see no more than 6,000 of them without a telescope.

Skywatching can teach you about celestial mechanics. By watching the movement of the planets and stars between, say, dinner and bedtime, you can observ how the Earth spins on its axis.

The Moon, planets, and stars traverse the night sky (and the Sun the day sky) from east to west because the Earth rotates from west to east.

By noting the changing positions of stars and planets over the course of two or more nights, you can witness how the Earth’s yearly motion around the Sun alters the positions of objects in the night sky. Stars near the sky’s equator rise (and set) four minutes earlier each night.

There’s some nifty magic to the math here: Stars rise (and set) two hours earlier each month, and 24 hours earlier each year – when the Earth has completed one orbit around the Sun.

The four-minute rule works for the planets Jupiter and Saturn, and often for Mars, Mosley says. Mercury and Venus orbit the Sun relatively quickly, though, and their paths are more complicated. Stars near the sky’s north pole remain above the horizon throughout the year, and the North Star remains in the same position – above the North Pole of the Earth.


Astronomers use degrees to measure the apparent size of objects in the sky and the distances between them.

An easy way to estimate the size of a single degree is to begin by visualizing the full Moon. It’s half a degree across, so two full Moons edge-to-edge equal a degree. The tip of your pinkie finger may just be about the right size when viewed along your outstretched arm.

To measure greater spans in the sky use your outstretched fist. From thumb to little finger, the average fist covers about 10 degrees.

A pair of binoculars is all you need to explore the night sky, and even the old naked eye can perform pretty well.

Apparent Brightness

The brightness of stars and other celestial objects is measured on a scale of apparent brightness. Smaller numbers are brighter (negative numbers are the brightest). The scale assumes dark skies.

Limiting Magnitude

Sky conditions can have a noticable effect on how much you can see even on a clear night. By using a list of stars of known magnitudes it is possible to determine the magnitude of the faintest visible stars on any particular night. Jeff has prepared a list of some suitable stars (downloadable as a PDF).

What’s Visible

Here are the planets and some of their moons, all visible with a good telescope, some visible with the naked eye. To find out which ones are favorably positioned for viewing tonight, see our Observing Officer’s monthly notes.
A scale of brightness

Mercury -1.9
Venus -4.4
Mars -2.0
Phobos +11.3
Diemos +12.4
Jupiter -2.7
Io +5.0
Europa +5.3
Ganymede +4.6
Callisto +5.6
Saturn +0.7
Rhea +9.7
Tethys +10.2
Titan +10.2
Iapetus,Dione +10.4
Enceladus +11.7
Mimas +12.9
Uranus +5.5
Neptune +7.8
Pluto +13.6