Astronomy BasicsDistances and VocabularyAngles and the Unit Circle

Powers of TenScientific Notation

What is Astronomy

Science: The quest for understanding.

Astronomy:

The oldest science.

The study of the universe from the smallest such as atomic reactions in stars to the largest such as galaxies and the evolution of the entire universe.

The View from Earth

Besides stars one can see:

NebulaeOur Galaxy (the Milky Way)GalaxiesPlanetsCometsSunMoon

Sagittarius and Scorpio

With telescopes many more objects can be seen.

The Celestial Sphere

It appears to rotate around us.

Extensions of Earth's poles and equator:

North celestial pole South celestial pole Celestial equator

Celestial sphere: An imaginary sphere around the Earth on which all heavenly objects appear to be located.

Meridian: An imaginary line that runs north to south, passing through the observer's zenith.

ConstellationsConstellations are areas of the sky

with stars in a particular pattern.

They are accidental patterns; stars are at different distances and not really related to each other.

Most patterns have been in use for 1000s of years and usually based on myths.

Today constellations are a useful way to divide the sky into regions.

How many official constellations are there?

Measuring the Positions of Celestial Objects

Angular separation: The angle between lines originating from the observer toward two objects.

60 arcminutes = 1 degree

60 arcseconds = 1 arcminute

At arm's length, roughly:• Fist = 10o across• Little finger = 1o across

Sun and Moon both = 0.5o wide

The angular separation of stars says nothing about their actual distances apart.

Your index finger, held at arms length, SUBTENDS about one degree.

The EclipticEcliptic: The path of the Sun in the sky. It is not the same as the celestial equator.

Zodiac: The constellations that the Sun passes through.

Altitude: The height of a celestial object measured at an angle above the horizon. The Sun has a higher altitude in summer than in winter in the Northern Hemisphere.

Solstice: Points on the celestial sphere where the Sun reaches its northernmost or southernmost points.

Northernmost = summer solstice around Jun 21st.Southernmost = winter solstice around Dec 21st.

Equinox: Point on the celestial sphere where the Sun crosses the celestial equator.

Vernal equinox is around Mar 21st.Autumnal equinox is around Sep 22nd.

Observations of Planetary MotionWithout a telescope you can see 5 planets: Mercury, Venus, Mars, Jupiter, Saturn.

They generally move eastward as the Sun and Moon do.

They remain within a few degrees of the ecliptic.

Mercury and Venus never appear too far from the Sun and are seen just after the Sun sets or just before it rises. Their elongations are limited.

Retrograde motion: The planets mostly move eastward, but at times some can change direction and move westward.

Figure 01.36A: Mars’ retrograde motion in 2003. Courtesy NASA/JPL-Caltech.

Units of Distance in Astronomy

Astronomical unit (AU): Used to measure distances within the solar system. It is equal to the average distance between the Earth and the Sun, which is about 150 million km.

Light-year: Used for longer distances. It is the distance that light travels in one year, which is about 9.6 trillion km.

What is a ParSec?

The Scale of the UniverseThe tremendous differences of sizes in the universe can be represented with scientific notation.

The use of powers-of-ten notation makes it much easier to describe the sizes of astronomical objects.