The Sky

Astronomy: Horizons10th edition

Michael Seeds

The Sky

• The night sky is the rest of the universe as seen from our planet.– When you look up at the stars, you look out

through a layer of air only a few hundred kilometers deep.

– Beyond that, space is nearly empty, and the stars are scattered light years apart.

The Sky

• As you read this chapter, keep in mind that you live on a planet.– Stars are scattered in the void all around you,

most very distant and some closer.– Earth rotates on its axis once a day—and that

makes the sky appear to revolve around you in a daily cycle.

– Not only does the sun rise in the East and set in the West, but so do the stars.

The SkyConstellations

• All around the world, ancient cultures celebrated heroes, gods, and mythical beasts by naming groups of stars after them—constellations.

The Sky

• The constellations named within Western culture originated in Mesopotamia over 5,000 years ago.

• Other constellations were added by Babylonian, Egyptian, and Greek astronomers during the classical age.– Of these ancient constellations, 48 are still in use.

Constellations

The Sky

• In 1928, the International Astronomical Union established 88 official constellations with clearly defined boundaries.– A constellation now represents not a group of stars but

an area of the sky.

Constellations

The Sky

• Apart from the 88 official constellations, the sky contains a number of less formally defined groupings called asterisms.– The Big Dipper is a well-known asterism that is

part of the constellation Ursa Major (Great Bear).

Constellations

The Sky

• The constellation names come from Greek versions translated into Latin, the language of science from the fall of Rome to the 19th century.

The Names of the Stars

The Sky

• Most star names come from ancient Arabic, though have been altered much by the passing centuries.– The name of Betelgeuse, the bright red star in Orion,

comes from the Arabic yad al-jawza, meaning ‘armpit of Jawza (Orion).’

– Names such as Sirius (the Scorched One) and Aldebaran (the Follower of the Pleiades) are beautiful additions to the mythology of the sky.

The Names of the Stars

The Sky

• Another way to identify stars is to assign Greek letters to the bright stars in a constellation in the approximate order of brightness.– Thus, the brightest

star is usually designated α (alpha), the second brightest β (beta), and so on.

The Names of the Stars

The Sky

• The ancient astronomers divided the stars into six classes. – The brightest were called first-magnitude stars and

those that were fainter, second-magnitude.– The scale continued downward to sixth-magnitude

stars—the faintest visible to the human eye. – Thus, the larger the magnitude number, the fainter

the star.– This makes sense if you think of the bright stars as

first-class stars and the faintest stars visible as sixth-class stars.

The Brightness of Stars

The Sky

• The faintest stars you can see with your unaided eyes are about sixth magnitude.

• If you use a telescope, you will see stars much fainter.– Thus, the scale has also been extended to include

numbers larger than sixth magnitude to include fainter stars.

The Brightness of Stars

The SkyThe Brightness of Stars

• These numbers are known as apparent visual magnitudes (mv).

• They describe how the stars look to human eyes observing from Earth. – Although some stars emit large amounts of infrared or

ultraviolet light, humans can’t see it, and it is not included in the apparent visual magnitude.

– The subscript ‘v’ stands for ‘visual’ and reminds you that you are including only light you can see.

The Sky

• Another problem is the distance to the stars.– Very distant stars look fainter and nearby stars

look brighter. – Apparent visual magnitude ignores the effect of

distance and informs you only how bright the star looks as seen from Earth.

The Brightness of Stars

The Sky

• Nonastronomers sometimes complain that the magnitude scale is awkward.– Why would they think it is awkward?– How did it get that way?

Building Scientific Arguments

The Sky

• The bigger the magnitude number, the fainter the star.– That arose because ancient astronomers were not

measuring the brightness of stars but rather classifying them.

– First-class stars would be brighter than second-class stars.

Building Scientific Arguments

The Sky

• As you study the sky, notice three important points.

• One, the sky appears to rotate westward around Earth each day.– That is a consequence

of the eastward rotation of Earth.

– That produces day and night.

The Celestial Sphere

The Sky

• Two, astronomers measure distances across the sky as angles and express them as degrees, minutes, and seconds.

The Celestial Sphere

The Sky

• What you can see of the sky depends on where you are on Earth.– If you lived in Australia, you would see many

constellations and asterisms invisible from North America, but you would never see the Big Dipper.

– Alpha Centauri is in the southern sky and isn’t visible from most of the United States.

– You could just glimpse it above the southern horizon if you were in Miami, but you could see it easily from Australia.

The Celestial Sphere

The Sky

• Earth spins like a giant top, but it does not spin upright in its orbit—it is tipped 23.5° from vertical. – Earth’s large mass and rapid rotation keep its axis of

rotation pointed toward a spot near Polaris.– The axis would not

wander if Earth were a perfect sphere.

Precession

The SkyPrecession

• However, Earth, due to its rotation, has a slight bulge around its middle.

• The gravity of the sun and of the moon pull on this bulge, tending to twist Earth upright in its orbit.

The Sky

• The combination of these forces and Earth’s rotation causes Earth’s axis to precess in a conical motion, taking about 26,000 years for one cycle.

Precession

The Sky

• Over centuries, precession has dramatic effects. – Egyptian records show that 4,800 years ago the north

celestial pole was near the star Thuban (α Draconis). – The pole is now approaching Polaris and will be

closest to it in about 2100. – In about 12,000 years, the pole will have moved to

within 5° of Vega (α Lyrae).

Precession

The Sky

• The figure shows the path followed by the north celestial pole.

Precession

The Sky

• A circumpolar constellation is one that does not set or rise.

Building Scientific Arguments

The Sky

• Which constellations are circumpolar depends on your latitude.– If you live on Earth’s equator, you see all the

constellations rising and setting and there are no circumpolar constellations at all.

Building Scientific Arguments

The Sky

– If you live at Earth’s North Pole, all the constellations north of the celestial equator never set and all the constellations south of the celestial equator never rise.

– In that case, every constellation is circumpolar.

Building Scientific Arguments