Introduction to the Sky

1. Until recently (past 50 years), the night sky was a topic of wonder, awe, and religion.

- calendar for planting

- calender for religious observances

- navigation

- stories/culture/folklore

- astrology/predictions for future

Many functions and knowledge were relegated to priestly or ruling classes. Now, with the exception of a few professionals and a number of amateurs, the night sky is a big mystery.

2. Main Objects

A. Sun ~100 times the diameter of the Earth. Basis of the Day and Year.

1. Sidereal Day 23h 56m – based on movement back to same star.

2. Mean Solar Day – 24h on average (varies slightly), based on

observations of sun from day to day.

Difference in two is caused by Earth’s motion.

3. Year 365.25 days

B. Moon ~3/11 diameter of the Earth. Basis of the Month.

1. Sidereal Month 27.3d – based on movement back to same star.

2. Lunar or Synodic Month 29.5d – based full moon to full moon.

Often associated with fertility since one lunation approximates the

menstrual cycle of humans.

C. Stars. Thousands visible. Many patterns called “Constellations”. They appear

to remain fixed wrt one another because they are so far away. They only change patterns over thousands of years.

D. Planets. Means “wanderer”. These look like stars except are brighter, and move.

--Five can be observed by eye: Mercury, Venus, Mars, Jupiter, Saturn. The others

were discovered in the past ~200 years using telescopes and cameras.

3. Constellations and Stars

A. Different patterns of stars were given names by different cultures.

- patterns remain essentially unchanged for thousands of years.

B. Different constellations are visible at different times of year. Sunlight effectively prevents us from seeing half of the visible constellations (from any location).

C. Sun, Moon, and Planets all follow roughly the same path through the sky, called the Ecliptic. Constellations along the ecliptic are part of the Zodiac and form a part of the art of astrology. There are currently 12 “signs” or constellations of the Zodiac, but at one point there were 13.

(Which was the 13^th? – Starry Night Project).

4. Motions of Objects.

A. Sun

1. Travels across the sky once every 24h, E to W. Day length varies depending on observers latitude. Shorter daylight in winter, longer in summer. At Bloomsburg University (latitude 41.00°N, longitude 77.46° W) daylight ranges from a minimum of 9h to a maximum of 15h.

2. Path of the sun defines the Ecliptic.

3. Ecliptic path goes through same stars year after year.

4. Constellations along ecliptic define the Zodiac.

5. Because of the Earth’s tilt (23.5°), ecliptic is higher during the day in summer, lower in sky during day in winter.

B. Moon

1. Travels across the sky (Full Moon to Full Moon) every 29.5 days.

2. Path is close to Ecliptic, but can vary by ~5° N or S of this.

3. Moon Phases - waxing, waning, crescent, gibbous.

4. Moon ~same angular size as Sun (1/2 of degree). When new moon is

positioned on the ecliptic, we get a solar eclipse. When full moon is on the ecliptic, we get a lunar eclipse.

5. Moon always shows same face to Earth. It’s day is ~1 month.

C. Planets

1. Travel across the sky at different rates. Closest to sun travel fastest.

2. All on/near the Ecliptic.

3. Some, especially those inside the Earth, show phases. (Galileo)

D. Stars

1. Appear fixed. Only move on timescales of thousands of years. Most

constellations appear little changed from when the pyramid builders

observed them.

5. Coordinates in the Sky

A. Celestial Sphere – imaginary sphere surrounding the Earth.

B. Celestial Equator – projection of Earth’s equator on Celestial Sphere

C. Celestial Poles – projection of Earth’s spin axis (N/S Poles) on sphere.

D. Declination – displacement N or S of Celestial Equator. Analogous to Latitude.

E. Right Ascension – displacement E or W of First point of Aries. Analogous to

Longitude. First point of Aries defined later, but equivalent to Prime Meridian.

F. Ecliptic – Path of Sun drawn on Celestial Sphere. A great circle tilted 23.5° from Celestial Equator (because of the Earth’s tilt). It crosses the C.E. at two points – when sun is at Spring and Autumn Equinoxes. Spring Equinox is First point of Aries, and is the point when the sun crosses the equator ascending.