1 Intro To Astronomy

Intro to Astronomy

Space is Big For distances within our solar

system, we use a unit of distance known as the Astronomical Unit (AU).

1 AU is defined as the average distance from the Earth to the Sun, roughly 1.5 x 108 kilometers (93 million miles)

Really Big When dealing with objects outside

of our solar system, the AU is too small to be effective, so we use the light-year.

A light-year (ly) is defined as the distance a beam of light travels in one year.

1 ly = 10 trillion km (6 trillion mi)

For Comparison... It takes a beam of light roughly 8

minutes to travel from the Sun to the Earth

Proxima Centauri (the nearest star to us, after our Sun) is over 4 light years away

The Celestial Sphere Imagine the Earth at the center of

a clear, hollow globe with the stars glued to the inside.

Everything we use to navigate on Earth can be “copied” onto the Celestial Sphere (latitude, longitude, the equator, and the poles)

Angular Measurement A full circle contains 360 degrees 1o can be broken further into arc

minutes (60’ in 1o) Arc minutes can be broken again

into arc seconds (60” in 1’)

Angular Measures The Sun and Moon both cover an

area of about 0.5o – half the size of a finger held at arm’s length

At arm’s length, a hand spans about 15o (also the amount of sky covered by the Sun’s motion in one hour)

Celestial Coordinates Declination (dec) is the equivalent

of latitude on the Celestial Sphere dec is measured in degrees north

or south of the Celestial Equator

Celestial Coordinates (cont.) Right Ascension (RA) is the

longitude equivalent on the C.S. RA is measured in hours, minutes,

and seconds The Prime Meridian of RA is

wherever the Sun is on the C.S. at the vernal equinox (first day of spring)

Orbital Motion Solar Day: the time it takes the

Sun to return to a specific spot in the sky (24 hours)

Sidereal Day: the time it takes Earth to complete one full rotation in its orbit (23 hours, 56 minutes)

The 4 minute per day difference gives us leap years

Seasonal Changes Earth’s orbit around the Sun

causes us to see different constellations in the sky

The Zodiac The ecliptic is the Sun’s path along

the Celestial Sphere. The Zodiac is made up of the 12

constellations that the Sun travels through along the ecliptic.

Due to position, the constellation of your sign can only be seen 6 months before/after your birth month.

Seasonal Changes (cont.) Earth rotates on its axis, which is

tilted 23 ½ degrees to its orbit. On the Celestial Sphere, the

ecliptic is tilted the same 23 ½ degrees.

This tilt is what gives us the four seasons.

Four Seasons

Four Seasons (cont.) Vernal Equinox – March 21st Autumnal Equinox – September 21st

12 hours of night and day - everywhere

Summer Solstice – June 21st

Most sunlight of the year Winter Solstice – December 21st

Least sunlight of the year

Distance & Size We can triangulate the distance to

an object we can’t directly measure

Distance & Size (cont.) With really large distances,

triangulation less reliable. Rather than used a measure

baseline, we use the missing angle of the triangle, or parallax

Distance & Size (cont.) Try this:

Hold a pencil in front of your face and let your eyes focus on the wall. First close your left eye, and then open it and close your right eye.

The apparently difference in position of the pencil is parallax

Review By this point, you should be able to:

Describe the Celestial Sphere Use angular measurements to find

objects in space Explain the apparent motion of the Sun

and stars with the actual motion of the Earth

Explain how to gauge size and distance of faraway object

Motions of the Planets ‘Planets’ comes from the Greek

word: ‘planetes’ which means “wanderer”

As viewed from Earth, the planets of our solar system all exhibit retrograde motion

Like the Moon, planets are visible because of reflected sunlight

The Geocentric Universe Until the 16th century, astronomers

believed that the Earth was the center of the universe

As a result, everything (the Sun, Moon, planets and stars) revolved around us

Astronomers tried everything to fit observations into this theory

The Heliocentric Model Nicholas Copernicus proposed the

idea of a Sun-centered universe in the 16th century

In fear of persecution, Copernicus kept his ideas secret until he died in 1543

Galileo & Kepler Galileo Galilei was the first

astronomer to use a telescope for observing the night sky

Using his telescope, he discovered: Sunspots Lunar terrain Moons orbiting Jupiter The phases of Venus

Eppur si muove For supporting Copernicus’ ideas,

Galileo was arrested and sentenced to death

He was spared the ultimate punishment and instead sentenced to house arrest for retracing his claims

Supposedly, he muttered “And yet, it moves” under his breath after he recanted

Kepler’s Laws

1. The planets revolve around the Sun in elliptical (not circular) paths

Perihelion: when a planet is closest to the Sun

Aphelion: when a planet is farthest from the Sun

Kepler’s Laws (cont.)

2. Planetary orbits sweep out equal areas of the ellipse in equal amounts of time

Kepler’s Laws (cont.)

3. The square of an orbital period is proportional to the cube of its semi-major axis

P2 (in Earth years) = a3 (in AU)

Kepler’s Laws (cont.)

Kepler’s Laws (cont.) During Kepler’s life, there were

only 6 known planets – those that can be seen without a telescope

Kepler’s 3rd Law works for Uranus, Neptune, and Pluto even though these were discovered after the 3rd law was written!

Solar System Dimensions Recall: the astronomical unit is

defined as the mean (average) distance from the Earth to the Sun

This was done because until recently, we lacked the technology to directly measure distances outside of Earth

Dimensions (cont.) Today, we use radar imaging to

directly measure the distance between planets

We send radio waves toward a nearby object (Venus, for example) and wait for the echo to come back

Multiply the round trip travel time by the speed of light and we calculate double the distance to the object

Example: Venus At its closest, Venus is 0.3 AU from

Earth A RADAR signal takes 300 s to

reach Venus and return to Earth 300,000 km/s * (300 s / 2) =

45,000,000 km = 0.3 AU Therefore, 1 AU = 150,000,000 km

Gravity The force due to gravity is

continuous and always attractive Unlike magnets, there is no

‘repulsive’ gravity

Gravity (cont.) All objects constantly exert a

gravitational force on each other – even you and me.

The force is only dependent on the mass of the objects and the distance between them

Newton’s Law of Gravitation

F = Gravitational Force G = Gravitational Constant = 6.67 x 10-11 N m2 / kg2

M1 = Mass of object #1 m2 = Mass of object #2 r = Distance between objects

221

r

mMGF

Important Notes The force decreases exponentially

with distance If you’re twice as far away, the force

is 22 times weaker (1/4 as strong) No matter how big r gets, the force

never reaches zero (gravity exerts an effect everywhere)

Example: The Lunar DietHow much would you weigh on the Moon? F = Force or Weight G = Gravitational Constant = 6.67 x 10-11 N m2 / kg2

M1 = Mass of the Moon (7.3477×1022 kg) m2 = Mass of you (~70kg) r = 1,737,000 m (Moon radius)

11300 m)(384,403,0

kg)(70kg) 10)(7.3477 10 x 6.67(2

22-11

W

Example (cont.) This compares to a weight on Earth

of:W = (70 kg) * (9.8 m/s2) = 686 N

Or, roughly, you’d weigh 1/6 as much on the Moon