CHAPTERCHAPTER 2CHAPTER2 2 ||| THETHE SKYTHE SKYSKY 2299
Summary
▶ Astronomers divide the sky into 88 constellations (p. 12). Although
the constellations originated in Greek and Middle Eastern mythology,
the names are Latin. Even the modern constellations, added to fi ll in the
spaces between the ancient fi gures, have Latin names. Named groups of
stars that are not constellations are called asterisms (p. 13).
▶ The names of stars usually come from ancient Arabic, though modern
astronomers often refer to a star by its constellation and a Greek letter
assigned according to its brightness within the constellation.
▶ Astronomers refer to the brightness of stars using the magnitude scale
(p. 14). First-magnitude stars are brighter than second-magnitude
stars, which are brighter than third-magnitude stars, and so on. The
magnitude you see when you look at a star in the sky is its apparent
visual magnitude, mv (p. 15), which includes only types of light
visible to the human eye and does not take into account the star’s
distance from Earth.
▶ (^) Flux (p. 15) is a measure of light energy related to intensity. The
magnitude of a star is related directly to the fl ux of light received on
Earth and so to its intensity.
▶ (^) The celestial sphere (p. 18) is a scientifi c model (p. 17) of the sky,
to which the stars appear to be attached. Because Earth rotates east-
ward, the celestial sphere appears to rotate westward on its axis.
▶ (^) The north and south celestial poles (p. 18) are the pivots on which
the sky appears to rotate, and they defi ne the four directions around
the horizon (p. 18): the north, south, east, and west points (p. 18).
The point directly over head is the zenith (p. 18), and the point on
the sky directly underfoot is the nadir (p. 18).
▶ (^) The celestial equator (p. 17), an imaginary line around the sky above
Earth’s equator, divides the sky into northern and southern halves.
▶ (^) Astronomers often refer to distances “on” the sky as if the stars, sun,
moon, and planets were equivalent to spots painted on a plaster
ceiling. These angular distances (p. 19), measured in degrees, arc
minutes (p. 19), and arc seconds (p. 19), are unrelated to the true
distance between the objects in light-years. The angular distance
across an object is its angular diameter (p. 19).
▶ (^) What you see of the celestial sphere depends on your latitude. Much of
the southern hemisphere of the sky is not visible from northern
latitudes. To see that part of the sky, you would have to travel
southward over Earth’s surface. Circumpolar constellations (p. 19)
are those close enough to a celestial pole that they do not rise or set.
▶ (^) The angular distance from the horizon to the north celestial pole
always equals your latitude. This is the basis for celestial navigation.
▶ (^) Precession (p. 17) is caused by the gravitational forces of the moon
and sun acting on the equatorial bulge of the spinning Earth and
causing its axis to sweep around in a conical motion like the motion
of a top’s axis. Earth’s axis of rotation precesses with a period of
26,000 years, and consequently the celestial poles and celestial equator
move slowly against the background of the stars.
▶ (^) The rotation (p. 20) of Earth on its axis produces the cycle of day and
night, and the revolution (p. 20) of Earth around the sun produces
the cycle of the year.
▶ (^) Because Earth orbits the sun, the sun appears to move eastward along
the ecliptic (p. 21) through the constellations completing a circuit of
the sky in a year. Because the ecliptic is tipped 23.4° to the celestial
equator, the sun spends half the year in the northern celestial hemi-
sphere and half in the southern celestial hemisphere.
▶ (^) In each hemisphere’s summer, the sun is above the horizon longer and
shines more directly down on the ground. Both effects cause warmer
weather in that hemisphere and leave Earth’s other hemisphere cooler.
In each hemisphere’s winter, the sun is above the sky fewer hours than
in summer and shines less directly, so the winter hemisphere has colder
weather, and the opposite hemisphere has summer. Consequently the
seasons are reversed in Earth’s southern hemisphere relative to the
northern hemisphere.
▶ (^) The beginning of spring, summer, winter, and fall are marked by the
vernal equinox (p. 22), the summer solstice (p. 22), the autumnal
equinox (p. 22), and the winter solstice (p. 22).
▶ (^) Earth is slightly closer to the sun at perihelion (p. 23) in January and
slightly farther away from the sun at aphelion (p. 23) in July. This has
almost no effect on the seasons.
▶ (^) The planets move generally eastward along the ecliptic, and all but Uranus
and Neptune are visible to the unaided eye looking like stars. Mercury
and Venus never wander far from the sun and are sometimes visible in the
evening sky after sunset or in the dawn sky before sunrise.
▶ (^) Planets visible in the sky at sunset are traditionally called evening
stars (p. 24), and planets visible in the dawn sky are called morning
stars (p. 24) even though they are not actually stars.
▶ (^) The locations of the sun and planets along the zodiac (p. 25) are
diagramed in a horoscope (p. 25 ), which is the bases for the ancient
pseudoscience (p. 25) known as astrology.
▶ (^) According to the Milankovitch hypothesis (p. 26), changes in the
shape of Earth’s orbit, in its precession, and in its axial tilt can alter
the planet’s heat balance and cause the cycle of ice ages. Evidence
found in seafl oor samples support the hypothesis, and it is widely
accepted today.
▶ (^) Scientists routinely test their own ideas by organizing theory and
evidence into a scientifi c argument (p. 27).
Review Questions
- Why are most modern constellations composed of faint stars or located
in the southern sky? - How does the Greek-letter designation of a star give you clues to its
location and brightness? - From your knowledge of stars names and constellations, which of the
following stars in each pair is the brighter? Explain your answers.
a. Alpha Ursae Majoris; Epsilon Ursae Majoris
b. Epsilon Scorpii; Alpha Pegasus
c. Alpha Telescopium; Alpha Orionis - How did the magnitude system originate in a classifi cation of stars by
brightness? - What does the word apparent mean in apparent visual magnitude?
- What does the word visual mean in apparent visual magnitude?
- In what ways is the celestial sphere a scientifi c model?
- If Earth did not rotate, could you defi ne the celestial poles and
celestial equator? - Where would you go on Earth if you wanted to be able to see both the
north celestial pole and the south celestial pole at the same time? - Why does the number of circumpolar constellations depend on the
latitude of the observer? - Explain two reasons why winter days are colder than summer days.
- How do the seasons in Earth’s southern hemisphere differ from those
in the northern hemisphere? - Why should the eccentricity of Earth’s orbit make winter in Earth’s northern
hemisphere different from winter in Earth’s southern hemisphere?