First Night Out Series: Measuring Brightness In The Sky

May 15, 2015 Pedro Braganca

In 120 B.C. Hipparchus, a Greek astronomer, ranked the brightness of stars in the sky on a scale of one to six. He classified the brightest stars he could see as first magnitude and ranked the rest down to the faintest at sixth magnitude.

Astronomers still use this scale to measure the brightness of celestial objects, although it has since been modernized.

The Magnitude Scale

The magnitude scale is logarithmic, so a difference of one point in magnitude is equal to a difference in brightness of about 2.5 times.

The magnitude of stars in the Big Dipper and Little Dipper asterisms.  Credit: Starry Night software. — The magnitude of stars in the Big Dipper and Little Dipper asterisms. Credit: Starry Night software.

A magnitude-one star is about 2.5 time brighter than a magnitude-two star, and a hundred times brighter than a magnitude-five star.

The lower the magnitude, the brighter the object.

The brighter planets and stars have negative magnitudes. The sun, the brightest object in the sky, has a magnitude of -26, followed by a full moon at magnitude -12.6.

Objects with a magnitude of six or less can be seen without optical aid under ideal observing conditions away from all artificial light.

Where Do Objects Fit in the Scale?

The table below is a list of well-known celestial objects and roughly where they fall on the magnitude scale—some objects, such as Venus, vary in brightness. The magnitude values have been rounded.

	
		Object
		Mag
	
		Sun
		-26
	
		Full Moon
		-12.6
	
		Crescent Moon
		-6
	
		Venus (the brightest planet)
		-4
	
		Jupiter
		-2
	
		Sirius (the brightest star in the sky)
		-1
	
		Vega (the brightest star in the Summer Triangle)
		0
	
		Saturn
		+1
	
		Polaris (the North Star) and the Stars of the Big Dipper
		+2
	
		The Andromeda Galaxy
		+4
	
		Uranus and the Faintest Stars Visible with the Naked Eye
		+6
	
		Objects You Need Binoculars to See
		+7 and greater

A Few Handy Terms

Here are a few handy terms to keep in mind when reading about the appearance of celestial objects.

Luminosity

Luminosity is the intrinsic brightness of a star—compared to the sun—as it would appear if you were there in orbit around it, rather than viewing it from Earth. The sun's luminosity is 1. Sirius has a luminosity of 23 and Betelgeuse has a luminosity of 55,000.

Brightness

Brightness is the light given off by a celestial object as seen from Earth. Brightness depends on luminosity and the distance from the object.

Magnitude

Magnitude is a logarithmic brightness scale. Magnitude-one objects are 2.512 times brighter than magnitude-two objects, which are 2.512 times brighter than magnitude-three objects, and so on. The difference between magnitude one and magnitude five is one hundred times. The higher the magnitude, the fainter the object. The lower the magnitude, the brighter the object. The brightest stars have negative magnitudes.

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First Night Out Series: Measuring Distances In The Sky

May 15, 2015 Pedro Braganca

Measuring the distance from one star to another in the sky is easy when you master using your hands to measure the degrees between objects.

Hold your hand at arm's length:

The width of your little finger is about one degree—enough to cover the moon and sun, both of which are each half a degree across.
The width of the first three fingers side-by-side spans about five degrees.
A closed fist is about ten degrees.
If you spread out your fingers, the distance from the tip of your first finger to the tip of your little finger is 15 degrees.
If you spread out your fingers, the distance from little finger to thumb covers about 25 degrees of sky.

With a bit of practice, this hand system is endlessly useful when measuring your way around the sky.

Calibrating with the Big Dipper

Everyone's hands are slightly different, so you might want to practice and calibrate your own hand measurements using the Big Dipper.

Here are the rough distances from Dubhe to several other prominent Big Dipper stars:

  
		Dubhe to Merak
		5 degrees
	
		Dubhe to Megrez
		10 degrees
	
		Dubhe to Alioth
		15 degrees
	
		Dubhe to Mizar
		20 degrees
	
		Dubhe to Alkaid
		25 degrees

First Night Out Series: Finding Your Way Around The Sky

May 13, 2015 Pedro Braganca

The Big Dipper is a great starting point for learning the night sky. Being circumpolar, it never completely sets or dips below the horizon—it's visible in the night sky year-round!

The Big Dipper itself is not a constellation, but it resides in one called Ursa Major, the Great Bear, the third largest of the 88 constellations. The name originates from the dipper-shaped pattern formed by the seven main stars in the constellation.

To locate the Big Dipper, face north and look for the seven bright stars that dominate the sky in this direction—they should be easy to find. Depending on the time of year, the pattern formed by these stars appears in a difference orientation, but the shape is always the same:

In autumn, the dipper appears to be sitting flat.
In spring, the dipper is upside-down, spilling its contents.
In summer, it sits upright on its bowl.
In winter, it sits up on its handle.

The stars of the Big Dipper are a handy guide to other stars, constellations, and other thought-provoking objects that may be too faint to spot with the naked eye. Using well-known spots in the sky to find fainter ones is known as star hopping—think of it as an astronomical treasure hunt! And one of the easiest and coolest place to start is with the two end stars that form the front of the dipper's bowl—they point straight to Polaris, the North Star.

All the other stars in the sky seem to turn counterclockwise around Polaris. Polaris itself marks the end of the handle of another pattern, the Little Dipper in Ursa Minor, the Little Bear. If you find Polaris, you know which way is north.

Following the arc of the handle of the Big Dipper points to two of spring's brightest stars—Arcturus and Spica. With a bit of practice, it's surprisingly easy to imagine lines and arcs from star to star and hop from constellations you know to those you're still learning.

The trick to successfully learning the night sky is to use easily recognizable star patterns to find the more difficult ones—just like we used the Big Dipper's stars to find Polaris.

Don't try to learn the entire sky on your first night out. Begin by learning the major constellations and then search out the more obscure patterns as the need and challenge arise.

Like riding a bicycle, once you know a constellation, it's hard to forget it.