# Astronomy and Space SciencePage 1

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## Astronomy and Space Science

Astronomy Basics

Length, time, angles

Celestial sphere, star maps

Solar System

Orbital Motion of the Earth around the Sun

Geocentric models

Heliocentric models

Modern views

Q&A

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## Units of Length

1 ls = distance light travel in 1 second = 299792485 m ≈ 3x108 m

1 ly = distance light travels in 1 year ≈ 9.46x1015 m ≈ 1016 m

1 AU (astronomical unit) = mean distance between the Sun and Earth ≈ 1.49x1011 m

1 pc (parsec) = distance from which 1 AU extends 1 arcsec ≈ 3.26 ly ≈ 3.24x1016 m

1 Mpc = 106 pc ≈ 3.26x1022 m

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Examples

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## Angles

Angles are measured in degree (°), arcmin ('), arcsec("); radians (rad, or no unit).

1° = 60' = 3600"

1 rad = 180°/π ≈ 57.3°.

Small angle approximation: angle = arc length/distance

The apparent diameter of the Sun and the Moon are about 0.5°.

Resolution limit of a 4" telescope ≈ 1".

Note: Do not confuse arcsec with inch, both use the same symbol.

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Objects with Large Angular Sizes (roughly to scale)

Andromeda Galaxy (M31)

180’ x 63’.

M54, extragalatic

globular star cluster, 12’

M4, globular star cluster, 36’

Orion Nebula (M42), 85’ x 60’.

Moon, 30’.

Sun, 30’.

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More Examples

Hubble Deep Field, ≈ 1.5’.

Polaris A’s apparent size = 0.002”.

Polaris A to Polaris Ab is 0.2”;

Polaris A to Polaris B is 20”;

Polaris A to Dubhe ≈ 30°.

Io, Jovian satellite, 1”.

Crab Nebula

Supernova

remnant,

6’x4’.

Ring nebula,

planetary

nebula,

1.4’ x 1’.

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## Celestial Sphere

The celestial sphere is a hypothetical sphere centered at the center of Earth.

On the celestial sphere, stars are fixed, while the Sun and the planets moves slowly.

The celestial sphere rotates, thus most stars rise and fall daily.

The celestial poles and celestial equator are projections of the poles and equator on the Earth on to the celestial sphere.

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