ASTRONOMYBasic terms and concepts

Measuring distances

Astronomical unit (AU) is defined as the distance from the Earth to the Sun.

1 AU is about 150 million kilometers We measure the distances in our Solar

System using the Astronomical Unit For example: Mercury is 0.3 AU from the

SunNeptune is about 30 AU from the Sun

Measuring Distances

Beyond the Solar System the AU is not very useful, use light year.

One Lightyear is the distance that light travels in one Earth year.

1 ly = 300,000 km/s x 365.25 days/yr x 24 hr/day x 60 min/hr x 60 sec/min = 1013 km

We are a few light years from the nearest star

Celestial Sphere

Celestial Sphere

The Celestial Sphere is an imaginary dome on which all the stars are projected.

It appears like all the stars are at the same distance from Earth.

On Earth it is like a dome, the ground blocks the lower part of the sphere

From out in space, it would appear like a sphere with the Earth at the center.

Celestial Sphere

Celestial Sphere

The Celestial Sphere is a model that helps us to simplify the location and motion of the celestial bodies.

We can put a grid on the Celestial Sphere, much the same as lines on a globe. These can be used to locate stars and constellations

Coordinates of Celestial Sphere

Declination and Right Ascension

Lines of Declination are the lines that run East West (pink)

Use “dec” to indicate the declinationValues are from +90 degrees to -90 degrees

The lines of Right Ascension “RA” are the lines that run from pole to poleDivided into 24 segments (each segment is

called an hour)Coordinated given in hours, minutes,

seconds.

Declination and Right Ascension

The point 0h, 0m, 0s is where the Sun’s path crosses the celestial equator.

The Celestial equator is the 0 degree of declination

The path of the Sun across the CS is known as the Elliptic

Elliptic

Elliptic

The belt shaped region surrounding the Elliptic passes through the twelve constellations of the Zodiak

Zodiak means little animals The position of the sun at any time of

the year is related to the constellation of the Zodiak.