Warming evenings and a night sky full of stars is a good time to learn the ABC’s of stargazing.
How do you describe the brightness of a star? And its location compared to other stars and permanent position in the night sky? This quick tutorial will also give you a sense of size of our Moon, as well as give you some stretching exercises for your hands and arm!
The human body is basically the same proportion in hand size, arm length and finger size no matter if your petite small or extralarge. So we can us our hands, fist and fingers as a reference for size or distance across the sky.
is for the angular size in degrees objects are from each other.
Of course a circle is 360 degrees, and from horizon to horizon is 180 degrees. Directly overhead is called zenith, and it 90 degrees from the horizon. Half way from the horizon and zenith is, of course, 45 degrees. These are basic reference points for saying “Jupiter is about 40 degrees above the southern horizon in Libra.” You can visualize it being about half way up in the sky from horizon to overhead.
Lots of astronomy calendars and monthly guides talks about “Neptune is 2 degrees above the Moon,” or “Venus is 20 degrees above the horizon at sunset.” How do you visualize those distances?
Well, just hold your arm out and spread your hand. The amount of sky covered from your thumb to your pinky finger is about 20 degrees, give or take a few degrees. Now clinch your fist. That is about 10 degrees of sky covered at arm’s length. Look at your index finger at arm’s length and that covers about 2 degrees of sky.
And your outstretched pinky finger is 1 degree of sky coverage, always enough to cover up the Moon!
hough large in minds, the Moon is only one-half degree across. Though it look huge when rising or setting against the mountains, trees or buildings, that is what scientists call a “Moon illusion” in our minds. You can always cover the
Full Moon with your outstretched pinky finger, whether it is rising, setting or directly overhead.
is for Brightness.
Though a little confusing at first the astronomy scale for brightness is a logarithmic scale where the lower the number the brighter, the higher the number the fainter. It is an arbitrary value of 2.5 times between each “magnitude.” The logarithm goes like this: 1st magnitude is 2.5 times brighter than 2nd magnitude; a 1st magnitude star is 6.25 times brighter (2.5 x 2.5) than a 3rd magnitude; a 1st magnitude star is 15 times brighter than a 4th magnitude (6.25 x 2.5); and so forth.
The faintest stars we can see is 6th magnitude—and that’s in a light pollution-free location, your eyes fully adapted to the dark. Under those optimum conditions there are about 6,000 stars in the Northern Horizon visible to the naked eye.
There about 20 stars that are 1st magnitude or brighter. Those brighter are given 0 magnitude or negative numbers, some with decimal fractions. The brightest star, Sirius, is minus -1.5 mag. The Sun, the obvious brightest object in our skies, is a minus -26th magnitude!
The second brightest object in the sky is our Moon. It is a minus -12th magnitude at full phase. Landing in third place on our magnitude scale is always planet Venus. Its global cloud deck reflects sunlight like a mirror, and it is always at least -3 mag., sometimes getting close to -4 mag. as it swings around the Sun and our evening and morning skies.
Jupiter at -2.5 mag. is usually fourth on the list of bright objects, but every two years Mars brightens up for a few weeks in July 2018 at -2.8 to rival ol’ Jove. By December Mars will be bright at 0 magnitude, but not the ruby red beacon it will be this summer.
Lots of serious telescopes can see down 12 to 14th mag. with the naked eye. With photography objects can be seen as faint as 18th magnitude by amateur astronomers. The world’s largest telescopes photograph down to 24th magnitude. The Hubble Space Telescope tops them all at around 31st magnitude.
is for Coordinates.
Like a globe of Earth with latitude for the north and south directions and longitude for the east and west, the sky is thought of as a celestial globe that also has coordinates. The north and south lines are called declination, and the east and west are called right ascension.
The latitude on Earth, called declination in the sky, are both expressed in degrees. The equator is 0 degrees, and the poles are 90 degrees. Longitude on Earth is also expressed in degrees, with 0 degree being an arbitrary point chosen in Greenwich, England winding around the world. In the sky, the right ascension coordinates have been divided into the 24 hours of the clock, as time moves the stars under that steady rhythm. The arbitrary starting point for the sky goes north and south through Cassiopeia, Pegasus and Cetus the Whale, called 0 hour Right Ascension.
All objects in the sky have coordinates, like towns on Earth. And today’s popular “go-to” telescopes are programed to automatically go to those coordinates.
Look at the Big Dipper rising in the northeast this spring. It’s famous pointer stars, the two outside the bowl, are 5 degrees apart, the bowl is 10 degrees across, your extended fist fitting right in between the four stars. And your hand span will cover the 20-degree length of the three stars of the bowl. Six of its seven stars are 2nd magnitude, the other 3rd mag. And one of its deep sky celestial treasures, the Owl Nebula near the bottom bowl star, has the coordinates 11 hours, 15 minutes Right Ascension, and +54 degrees, 55 minutes Declination.
Get out in the fresh smells of a Spring evening and practice you’re ABCs of astronomy—Angular size, Brightness and Coordinates. You will begin to comprehend some of the language of stargazing, and hopefully understand a bit more about our amazing Universe.