# Geometric UniversePage 1

#### WATCH ALL SLIDES

Slide 1

Astronomy and Space Science I

Dr. Hoi-Fung Chau

and

Dr. Alex Tat-Sang Choy

Jointly Organized by

Hong Kong Space Museum

HKU Physics Department

Co-organized by

CDI of EDB

Slide 2

## 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.

Slide 8

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’.

Slide 10

## Celestial Sphere

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