Sky Maps

Before we dive into reading Star Maps, let’s revisit some of the concepts at https://www.awestronomy.com/rotation-of-the-night-sky/

Now that we are caught up with the basics, let’s learn a few key concepts before exploring Star Maps.

The Ecliptic and the Zodiac

The celestial equator goes all the way around the celestial sphere directly above the Earth’s equator. There’s another circle that goes all the way around the sky. It’s called the ecliptic, and it’s tilted with respect to the equator by 23.5º. The ecliptic is the imaginary circle on the sky that marks the annual path of the Sun. It’s tilted because the Earth itself is tilted relative to its orbit around the Sun by 23.5 degrees (see above). If the Earth was not tilted in its orbit around the Sun, the celestial equator and ecliptic would be the same circle.

The tilt of the Earth’s axis, showing the plane of the ecliptic inclined to the celestial equa-tor and the position of the equinoxes and solstices.

Because of Earth’s tilt, the Sun appears highest in the sky relative to the celestial equator when the Earth is at one position in its orbit. This happens on or about June 21, and we call this the summer solstice (in the northern hemisphere). When the Earth is at the opposition side of its orbit in December the Sun is at its lowest point in the sky relative to the celestial equator. This is the winter solstice. Between the two, the Sun is right on the celestial equator. These are spring and autumnal equinoxes when spring and autumn begin. The equinoxes and solstices are four points on the ecliptic.

What’s more, since all the planets lie near the same flat plane around the Sun, the ecliptic also marks the path of the planets around the sky as they revolve around the Sun. So every planet, the Sun, and even the Moon, are always found on or very close to the ecliptic during the year.

As it turns out, the great circle of the ecliptic passes through 12 formal groups of stars called constellations. This group of constellations is called the zodiac, and it includes Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces. (The ecliptic actually passes through a 13th constellation, Ophiuchus, but it is not included in the zodiac because ancient astrologers regarded the number 13 as unlucky).

How To Read A Star Chart

When you’re just starting out you need a good basic star chart that shows you where to find the bright stars and main constellations at a particular time and place. At first, star charts are a little confusion. So here’s how to read a star chart.

Here is a basic star chart showing the sky on April 15 at 9 p.m. from 40 degrees north latitude.

A sample star chart showing the northern sky at 9 p.m. on April 15 (click to enlarge)

The chart above tries to represent a hemispherical sky on a flat surface. The edge of the chart represents the horizon, and the center of the chart is supposed to represent the zenith (the point directly overhead) at 40 degrees north. East and west are reversed compared to an map of the Earth, but they will point in the right directions when you raise the map over your head.

As you learn the night sky, you will want to star charts. To read a star chart, here’s what you have to do…

Find a location that’s isolated from street and house lights. Stray light will make it harder for you to see fainter stars. Also, for the same reason, try to avoid nights with a full moon or too much haze.
Once you go outside, give your eyes 5 or 10 minutes to become adapted to the dark. And to see the star charts, use a red LED flashlight or a white flashlight covered with red plastic. The red light will preserve the sensitivity of your eye for night viewing. (More about this on Day 5).
Pick a direction to face, say, South, and rotate the chart so South is at the bottom. Now raise the chart overhead. The directions on the chart will now correspond to the directions in the sky.
Don’t try to take in the whole sky at once. Choose a quarter of the map, preferably one with several bright stars or a large well-known constellation like Orion or Ursa Major (of which the Big Dipper is a part). Now, look up at the quarter of the sky that corresponds to the quarter of the map. Make a connection with what you see in the sky with what you see on the map. Take your time… it’s a little strange and overwhelming at first.
Learn a few more stars at a time… don’t rush. Once you’ve identified a few bright stars and constellations, move from what you know to what you don’t know. Once you’ve learned most of a quarter of the sky, move to another quarter.
While the charts are set for 9 p.m. local time, they are still useful for an hour or two on either side. The stars will appear in about the same position, except for the stars near the horizon. After 3 hours, the stars will have turned 1/8 of the way around the sky. And after 6 hours, they will have turned 1/4 of the way around the sky.
If you see an out-of-place star near the ecliptic (and in one of the constellations of the zodiac), it’s almost certainly a planet. Since the planets move around in the sky almost daily, you will need to consult an almanac or website to figure out which planet you are seeing. Sky and Telescope is and especially good place to check. We also review the positions of the planets each month here at One-Minute Astronomer.

That’s all there is to it. Well, that and a whole lot of practice. Be patient, and savor your personal discovery of each new star and constellation.

Measuring The Sky

Many new stargazers have trouble understanding our reference to “degrees”, “arc minutes”, and “arc seconds” when talking about the separation of celestial objects. So here’s a primer on measuring angular distances.

Astronomers measure angular separation of objects in degrees. There are 360 degrees in a circle. And the angular separation of any point on the horizon and the point directly overhead (the zenith) is 90 degrees. Halfway from the zenith to the horizon is 45 degrees.

Smaller angles are a little trickier. But your hands and fingers are a remarkably accurate (and convenient) measuring tool. When you hold your hand at arm’s length, you can estimate angles like this:

Stretch your thumb and little finger as far from each other as you can. The span from tip to tip is about 25 degrees
Do the same with your index finger and little finger. The span is 15 degrees
Clench your fist at arms length, and hold it with the back of your hand facing you. The width is 10 degrees
Hold your three middle fingers together; they span about 5 degrees
The width of your little finger at arm’s length is 1 degree.

Now let’s go smaller. When you look through a telescope, you see a field of view of 1 degree or less… a very small slice of sky.

Astronomers measure angles smaller than 1 degree in arcminutes, or “minutes of arc”. There are 60 arcminutes in one degree, so 1 arcminute is 1/60 degree. The symbol for arcminutes is ‘. So the full Moon, for example, is about 30′ (thirty arcminutes) across. Coincidentally, so is the Sun.

Each arcminute is divided into 60 arcseconds, or “seconds of arc”. So 1 arcsecond is 1/60 arcminute and 1/3600 degree. The symbol for arcseconds is “. The face of Jupiter, which you can see this summer, is about 50″ across. A good optical telescope in steady skies can resolve down to about 1″ (one arcsecond).

How To Read Sky Coordinates

Some new subscribers are terrified by the coordinate system for the celestial sphere. But if you understand the concept of latitude and longitude on the Earth, you can understand their celestial equivalents. Here’s what you need to know to find things on a star map.

A Quick Review…

On maps of the Earth, latitude measures how far north or south of the equator a place lies. By convention, the equator has a latitude of zero degrees, the north and south poles have a latitude of 90° North and 90° South, respectively. Chicago has a latitude of 41.8° North; Sydney, Australia has a latitude of about 34° South.
Longitude measures how far east and west a place lies on the Earth’s surface. But how far east and west of what? By convention, the reference point of longitude is the great circle running through the earth’s poles and the Royal Greenwich Observatory in London, U.K. So Greenwich is at zero degrees longitude. Chicago, west of Greenwich, has a longitude of 88° West. Sydney, east of London, is at a longitude of 151° East.

The celestial sphere, showing the poles, equator, and celestial coordinates

Celestial Latitude: Declination

Now imagine the lines of latitude and longitude projected onto the sky. The celestial equator lies directly above the Earth’s equator, and the north and south celestial poles are above the Earth’s poles.
Imaginary lines of latitude and longitude are there as well. But in the sky, latitude is called declination. By convention, the celestial equator has a declination of 0 degrees. North and south of the celestial equator, declination is marked with a “plus” and “minus” sign. The star Vega, for example, has a declination of +39°. The southern star Achernar has a declination of about -57°.
Each degree is split into 60 smaller units called “minutes of arc”, marked by a ‘, and each minute is split into 60 “seconds of arc”, marked by a “. So the more precise declination of Achernar is -57° 14′ 12″. And Vega is at +38° 47′ 01″.

Celestial Longitude: Right Ascension

The celestial equivalent to longitude is called right ascension. It’s measured not in degrees but in “hours”, from 0h to 24h. Astronomers cooked up this arrangement long ago because the celestial sphere appears to turn once every 24 hours. With 24 hours in the full 360 degrees of sky, each hour corresponds to 15 degrees of angular distance. Like degrees, each hour is split into 60 minutes, and each minute into 60 seconds.
The right ascension of Achernar, for example, is 01h 37m 43s; Vega is at right ascension 18h 36m 56s.
By convention, the great circle with right ascension of 0 hours runs through a point in the constellation Pisces at which the ecliptic crosses the celestial equator, and right ascension increases going eastward.

Good To Know

The right ascension and declination of each star are fixed from day to day and year to year. But because the Earth is wobbling in space because of the gravitational influence of the Moon and Sun, the coordinates of celestial objects change over the course of decades. Every 50 years or so, star maps and star coordinates are updated. Current star maps are accurate as of the year 2000.

Stellar Magnitude: Understanding the Brightness of Stars and Planets

Astronomers use a numerical measure called “magnitude” to describe the brightness of stars, planets, and other objects in the night sky. Here’s how it works.

By convention, brighter objects have a smaller numerical value of magnitude than fainter objects. So a star with magnitude 4 is brighter than a star with magnitude 5, for example. It’s a little like a ranking system, where brighter stars are assigned a smaller number. Again, by convention, stellar magnitudes are defined so that an object with magnitude 1.0 is 100 times brighter than an object with magnitude 6.0. So each step of 1.0 in magnitude is the “fifth root” of 100, which is a factor of 2.512. That means a star of magnitude 3.0 is 2.512 times as bright as a star of magnitude 4.0, which is 2.512 times as bright as a star of magnitude 5.0, and so on. Try it yourself, if you have a calculator handy.

With your naked eye, you can see objects down to 6th magnitude; with a pair of 7×50 binoculars you can see down to magnitude 10.5 or so; and with an 8” telescope, perhaps magnitude 13.5. Using sophisticated cameras and software, the Hubble can detect objects to about 30th magnitude… about 4 billion times fainter than you can see with your eye.

For numerical convenience, an object brighter than 0th magnitude has a negative magnitude; the brightest star, Sirius is magnitude -1.4; the full moon is magnitude -13, and the Sun is a blazing -26th magnitude.

The “apparent” magnitude measures how bright a star appears in the sky, regardless of how bright is truly is.

“Absolute” magnitude is a measure of the true, intrinsic brightness of a star. It’s defined as the apparent magnitude of an object if it was 32.616 light-years away. While the sun has an apparent magnitude of -26, it has a modest absolute magnitude of 4.7. Deneb, the brightest star in Cygnus, has an absolute magnitude of -8.73, more than 250,000 times as bright as our Sun. But its apparent magnitude is only 1.25 because it’s so far away, roughly 3,200 light-years from Earth.

The ancient Greek astronomer Hipparchus developed the system of magnitude we use today back in 120 B.C. He used his system to catalog the brightness and position of 1,080 stars. In 1996, a European satellite named after Hipparchus created the most accurate catalog to date. It lists the precise positions of over 120,000 stars, and is available online to anyone who wants to use it.

The Brightest Stars

Here is a list of the 25 brightest stars in the night sky visible from Earth. From Sirius, the brightest star, to Shaula in the constellation Scorpius, this list of bright stars includes the apparent visual magnitude, which is a measure of the brightness you see with your unaided eye. The apparent brightness of a star is influenced by how bright it really is (its intrinsic brightness), which is measured by its absolute magnitude, and its distance. Both are listed here for each star.

The Sun is not included on this list. It has an apparent visual magnitud of -26.7 and an absolute magnitude of 4.7.

Star Names

Of all the stars in our galaxy, only 100 or so of the brightest stars have proper names. Rigel in Orion, Vega in Lyra, Altair in Aquila, all have names handed down from Greek, Roman, and Arab astronomers from antiquity.

The star Betelgeuse in Orion. It is also known as alpha Orionis, 58 Orionis, HR 2061, BD +7° 1055, HD 39801, FK5 224, HIP 27989, and SAO 113271

In the early 1600’s, just before the invention of the telescope, the astronomer Johann Bayer developed a system to name hundreds more stars using Greek letters. He usually named the brightest star in a constellation alpha (α), the second brightest beta (β), the third brightest gamma (γ), and so on through to the last letter omega (ω). Bayer’s system is still in use today. And it did not supersede the ancient names of the stars, so Vega in Lyra is also named α Lyrae, and the star Mintaka in Orion is called δ (delta) Orionis.

Of course, with only 24 letters in the Greek alphabet, Bayer’s system ran out of names. So more systems were devised. The British astronomer John Flamsteed numbered the stars in each constellation from west to east. Flamsteed cataloged up to a hundred or more stars in some constellations, though stars in the Bayer catalog were not given numbers. The nearby star 61 Cygni is an example of a star in Flamsteed’s catalog.

As more stars were discovered and mapped, more catalogs were developed. The Bonner Durchmusterung (BD) catalog, the Henry Draper (HD) catalog), and the Smithsonian Astrophysical Observatory (SAO) catalog are all examples, and you will come across these designations in star atlases. Nearly all of the stars you see with a backyard telescope will be listed in at least one of these catalogs.

Deep-Sky Catalogs

In the late 18th century, the French comet hunter Charles Messier became frustrated when he kept sighting faint, diffuse celestial objects he mistook for comets. To prevent confusion, he cataloged the positions of 103 of these objects. Armed only with a tiny telescope, Messier had no idea what these objects were. But more than 200 years later, his catalog is now the most well-known list of galaxies, stars clusters, and nebulae accessible with small telescopes.

The Great Orion Nebula, also known as Messier 42 (M42) or NGC 1976.

Objects in the Messier list are designated with an M and a number. The Crab Nebula in Taurus, for example, is M1. The Pleiades is listed as M45. And the lovely “Wild Duck” star cluster in Scutum is M11. Since Messier was based in the northern hemisphere, all the objects in the catalog are in the northern and near-southern sky, though many of the objects can be seen in populated areas of the southern hemisphere. Messier’s catalog was later expanded to 110 objects. With dark sky, you can see all 110 objects in a 3” or 4” telescope, though usually not all in one night. (Although there is a window in March when observers in some parts of the world can see all the Messier objects in one night… an astronomical endurance event known as a “Messier Marathon”).

Of course, there are far more than 110 sights to see in the night sky, and later astronomers compiled more extensive catalogs. J. Dreyer developed the New General Catalog (NGC), which contains the positions of 7,840 objects observed by William Herschel and others. Like the Messier catalog, the NGC lists galaxies, star clusters, and nebulae. Unlike the Messier catalog, not all objects are accessible with backyard scopes, though you can see hundreds or thousands of NGC objects, depending on your skies, your skill, and your telescope.

There are other catalogs, including the Index Catalogs (IC), which extend the NGC, the Collinder and Melotte catalogs of open star clusters, and E. E. Barnard’s catalog of dark nebulae. You will come across objects in these catalogs from time to time in your exploration of the night sky.