AstronomyPhysics 102 Goderya

Chapter(s): OnlineLearning Outcomes: 1,2,10,11,12

Scales of Size and Time

Astronomy deals with objects on a vast range of size scales and time scales.

Most of these size and time scales are way beyond our every-day experience.

Humans, the Earth, and even the solar system are tiny and unimportant on cosmic scales.

Earth Orbiting Around the Sun

In order to avoid large numbers beyond our imagination, we introduce new units:

1 Astronomical Unit (AU) = Distance Sun – Earth = 150 million km

The Solar System

Approx. 100 AU

The Solar Neighborhood

Approx. 17 light years

New distance scale:

1 light year (ly) =

Distance traveled by light in 1 year

= 63,000 AU = 1013 km

= 10,000,000,000,000 km

(= 1 + 13 zeros)

= 10 trillion km

Nearest star to the Sun:

Proxima Centauri, at a distance of 4.2 light years

The Milky Way Galaxy

Diameter of the Milky Way: ~ 75,000 ly

Finding objects in the sky

• Constellations

Source: Jodrell Bank Observatory

Finding objects in the sky

• Orion Nebula

Source: Jodrell Bank Observatory

Constellations 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:

OrionBetelgeuze

Rigel

Betelgeuse = OrionisRigel = Orionis

The Magnitude Scale

First introduced by Hipparchus (160 - 127 B.C.):

• Brightest stars: ~1st magnitude

• Faintest stars (unaided eye): 6th magnitude

More quantitative:

• 1st mag. stars appear 100 times brighter than 6 th mag. stars

• 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!)

Betelgeuse

Rigel

Magnitude = 0.41 mag

Magnitude = 0.14 mag

The Magnitude Scale (Example)

Magn. Diff. Intensity Ratio

1 2.512

2 2.512*2.512 = (2.512)2 = 6.31

… …

5 (2.512)5 = 100

For a magnitude difference of 0.41 – 0.14 = 0.27, we find an intensity ratio of (2.512)0.27 = 1.28.

The Magnitude Scale

Sirius (brightest star in the sky): mv = -1.42Full moon: mv = -12.5

Sun: mv = -26.5

The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects):

Apparent Motion of The Celestial Sphere

Precession

The Sun’s gravity is doing the same to Earth.

The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.

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

PrecessionAs a result of precession, the celestial north

pole follows a circular pattern on the sky, once every 26,000 years.

It will be closest to Polaris ~ A.D. 2100.

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.

The Sun and Its Motions

Earth’s rotation is causing the day/night cycle.

The Sun and Its Motions

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

Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere.

Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations.

The Seasons

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

The Seasons

The Seasons

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.

Light from the sun

Steep incidence → Summer

Shallow incidence → Winter

The Seasons are only caused by a varying angle of incidence of the sun’s rays.

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)

The Phases of the MoonFrom Earth, we see different portions of the Moon’s surface lit by the sun, causing the phases of the Moon.

Lunar EclipsesEarth’s shadow consists of a zone of partial shadow, the Penumbra, and a zone of full shadow, the Umbra.

If the moon passes through Earth’s full shadow (Umbra), we see a lunar eclipse.

If the entire surface of the moon enters the Umbra, the lunar eclipse is total.

A Total Lunar Eclipse

A Total Lunar EclipseA total lunar eclipse can last up to 1 hour and 40 min.

During a total eclipse, the moon has a faint, red glow, reflecting sun light scattered in Earth’s atmosphere.

Solar Eclipses

The sun appears approx. as large in the sky (same angular diameter ~ 0.50) as the moon.

When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse.

Total Solar Eclipse

Prominences

Chromosphere and Corona

Diamond Ring Effect

Earth and Moon’s Orbits Are Slightly Elliptical

Sun

Earth

Moon

(Eccentricities greatly exaggerated!)

Perihelion = position closest to the sun

Aphelion = position furthest away

from the sun

Perigee = position closest to Earth

Apogee = position furthest away from Earth

Conditions for Eclipses

A solar eclipse can only occur if the moon passes a node near new moon.

The moon’s orbit is inclined against the ecliptic by ~ 50.

A lunar eclipse can only occur if the moon passes a node near full moon.

Conditions for EclipsesEclipses occur in a cyclic pattern.