The Sky Chapter 2

Outline I. The Stars A. Constellations B. The Names of the Stars C. The Brightness of Stars D. Magnitude and Intensity II. The Sky and Its Motion A. The Celestial Sphere B. Precession III. The Cycles of the Sun A. The Annual Motion of the Sun B. The Seasons IV. The Motion of the Planets A. The Moving Planets B. Astrology V. Astronomical Influences on Earth's Climate A. The Hypothesis B. The Evidence

Constellations

In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures.

Constellations Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

Naming Stars Stars are named by a Greek letter (  ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Betelgeuse =  Orionis Rigel =  Orionis Orion is the notable exception to this rule. Betelgeuse and Rigel are incorrectly named.

Constellations Orion Betelgeuse Rigel

Constellations The stars of a constellation only appear to be close to one another Usually, this is only a projection effect. The stars of a constellation may be located at very different distances from us.

Star Brightness

The Magnitude Scale First introduced by Hipparchus ( B.C.): Brightest stars: ~1 st magnitude Faintest stars (unaided eye): 6 th magnitude Larger Magnitude = Fainter Object !

Intensity Ratio Intensity Ratio: For each order of magnitude in difference, the object is times brighter or dimmer

The Magnitude Scale Difference in Magnitude Intensity Ratio (times brighter) 1(2.512) 1 = 2.5x 2(2.512) 2 = 6.3x 3.5(2.512) 3.5 = 25.1x 4.871(2.512) = 88.8x 5(2.512) 5 = 100.0x

The Magnitude Scale Betelgeuse Rigel Magnitude = 0.41 mag Magnitude = 0.14 mag How much brighter is Rigel than Betelgeuse?

The Magnitude Scale The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Sirius (brightest star in the night sky): m v = Full moon: m v = Sun: m v = -26.5

The Magnitude Scale

Celestial Sphere

The Celestial Sphere Zenith = Point on the celestial sphere directly overhead of your current position. Nadir = Point on the celestial sphere directly underneath (never visible!) Celestial equator = projection of Earth’s equator onto the celestial sphere North celestial pole = projection of Earth’s north pole onto the celestial sphere

The Celestial Sphere

From geographic latitude l (in the northern hemisphere), you see the celestial north pole l degrees above the northern horizon From geographic latitude - l (southern hemisphere), you see the celestial south pole l degrees above the southern horizon. Celestial equator culminates 90º – l above the horizon.

The Celestial Sphere Atlanta: l ≈ 33.7º The Celestial South Pole is not visible from the northern hemisphere. Horizon North Celestial North Pole

The Celestial Sphere At latitude 90-degrees (North Pole) the Celestial North Pole is 90- degress above the horizon (straight up)

The Celestial Sphere At latitude 0-degrees (the equator) the Celestial North Pole is located 0-degrees above the horizon (at the horizon).

The Celestial Sphere At a latitude of 30-degrees (Florida pan-handle) the Celestial North Pole is located at 30-degrees above the horizon (a little lower than half way to zenith).

Apparent Motion

Precession At left, gravity is pulling on a slanted top. Wobbling around the vertical.

Precession The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical with respect to the Ecliptic takes about 26,000 years and is called precession.

Precession As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years.

Precession It will be closest to Polaris ~ A.D There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, it will be close to Vega in the constellation Lyra.

iClicker Question In what direction would you look this evening to find Scorpio? A. North B. South C. East D. West E. Scorpio is below the horizon at night.

Scorpio Objects Butterfly ClusterPtolemy Cluster

Scorpio Objects M4M80

iClicker Question Identify the brightest star in Scorpio A. Aldeberan B. Antares C. Altair D. Arcturus E. Atria

Antares “Heart of the Scorpion” Red Supergiant – 800 times larger than our sun Magnitude: ~1.0 ± 0.1

Motion of the Sun

iClicker Question If you were standing on the Earth’s equator, where would Polaris appear to be? A. Zenith B. Celestial Equator C. Horizon D. Nadir E. Directly Overhead

The Sun and Its Motions Earth’s rotation is causing the day/night cycle.

The Sun and Its Motions Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations.

iClicker Question How many times brighter would a magnitude -0.5 star appear than a +1.0 star? A. 1.5x B x C x D. 3.98x E. The +1.0 star is brighter

The Sun and Its Motions The Sun’s apparent path on the sky is called the Ecliptic.

The Seasons

Earth’s axis of rotation is inclined vs. the normal to its orbital plane by 23.5°, which causes the seasons.

iClicker Question What is the faintest magnitude star seen by the unaided eye? A. -2 B. 2 C. 4 D. 6 E. 8

The Seasons The Seasons are only caused by a varying angle of incidence of the sun’s rays. They are not related to Earth’s distance from the sun. In fact, Earth is slightly closer to the sun in (northern- hemisphere) winter than in summer.

The Seasons Northern summer = southern winter Northern winter = southern summer

The Seasons Earth’s distance from the sun has only a very minor influence on seasonal temperature variations. Sun Earth in July Earth in January Earth’s orbit (eccentricity greatly exaggerated)

Planetary Motion

The Motion of the Planets The planets are orbiting the sun almost exactly in the plane of the Ecliptic. Jupiter Mars Earth Venus Mercury Saturn The Moon is orbiting Earth in almost the same plane (Ecliptic).

The Motion of the Planets All outer planets (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto) generally appear to move eastward along the Ecliptic. The inner planets Mercury and Venus can never be seen at large angular distance from the sun and appear only as morning or evening stars.

The Motion of the Planets Mercury appears at most ~28° from the sun. It can occasionally be seen shortly after sunset in the west or before sunrise in the east. Venus appears at most ~46° from the sun. It can occasionally be seen for at most a few hours after sunset in the west or before sunrise in the east.

Factors Affecting Climate

Astronomical Influences on Earth’s Climate Factors affecting Earth’s climate: Eccentricity of Earth’s orbit around the Sun (varies over period of ~ 100,000 years) Precession (Period of ~ 26,000 years) Inclination of Earth’s axis versus orbital plane Milankovitch Hypothesis: Changes in all three of these aspects are responsible for long-term global climate changes (ice ages).