Constellations, Motion of the Earth, and Celestial Coordinates

--- Read Sections 1.1 through 1.5 of Chaisson ----

Additional Information

• Star Charts
• Constellations and Star Designations
• Angles and Angular Measurement

Read Special Boxes
• Scientific Notation - p8 -We will be using large and small numbers extensively in this course, so you must know the numerically operations on numbers in the scientific notation.
• Astronomical Measurement - p13 - We will be using metric units exclusively in Astronomy. Units such as feet, miles, and pounds are not used by astronomers. New units will be used for convenience: examples are to express masses in units of Solar or Earth masses; distances are in large units such as the distance from the Sun to the Earth, or the distance that light travels in one Earth year.
• Angular Measure - p14 - In astronomy, most measurements are of angles, not distances, so you must be familiar with angular units. We will be using degrees, minutes, and seconds of arc, as well as hours, minutes, and seconds of time. In addition the radian unit which is not mentioned in the textbook will be used (defined later).

Figures of Note:
• Figure 1.7 - Definition of the Celestial Sphere, Celestial Equator, North Celestial Pole (NCP) and South Celestial Pole (SCP)
• Figure 1.9 - Illustrates the Difference between the Solar and Siderial Day (Be sure you understand this diagram)
• Figure 1.10 - Shows how the Ecliptic is defined and the location of the equinoxes and solstices.
• Figure 1.11 and 1.12 - Illustrates the changes of constellations in the sky from season to season.
• Figure 1.13 - Defines the Celestial Coordiate System (Besure you understand this diagram)

Sky Charts

There are useful Sky Charts in the Back of your Textbook S-1 through S-4. These charts show 6 hours of right ascension centered on the celestial equator. These are useful for finding constellations and star using right ascension and declination. Unfortunately, no polar charts are included.

See the Charts in the Observer's Handbook at the back. Read the introductory page. These charts are centered on the local zenith for 45 north latitude. The borders represent the horizon and the north, south, east and west points are labeled. Constellations south of 45s are not shown on these charts.

A Windows sky program called the Earth Centred Universe is available on the AXE network - its path is
x:\public\apps\ecu\ecu.exe.
This program will generate sky charts for any location on the earth for any date and time. It will also plot positions of the Sun, Moon, planets, comets and asteroids.

Other Information of Value

The Constellations

Refer to the Observer's Handbook pages 182-183 (1996) for a list of all 88 constellation, the pronunciation of their names and genetive case plus their English meaning.

Greek Alphbet and Bayer system of star naming

The brighter stars have been given names through out history by various cultures. We are familiar with stars like Sirius, Vega, Deneb, etc. But Bayer used the Greek Alphabet to label the brightest stars in the constellations. Usually the brightest is labeled alpha, the next brightest as beta, the third as gamma etc. This is not alway true as in the case of Ursa Major (Big Dipper). The Online Star Charts only have the stars label with the Bayer system. There are other systems that label the stars that are dimmer. We will talk about these in the next term.

Angular Measurement of Size

The Radian is an important unit of angular measure that equals 57.3 degrees. It's importance is that by putting the angle in radians, the arc length defined at the rim of the circle is equal to the angle time the circle radius.

When angles are small (which is very common in astronomical measurements) the chord length at a distance may easily be calculated from the angle subtended. This is possible because for small angles the chord length equals the arc length. As an example, the difference between the chord length subtended by 10 degrees is only different from the arc length subtended by 0.1 percent. The method is sometimes called the skinny triangle. When very small angles are utilized, it is useful to measure them in " arc rather than radians. One radian equals 206,265 " arc.

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