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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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Slide 3

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

Slide 4

Examples

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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 7

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’.

Pleiades, open star cluster, 180’.

Moon, 30’.

Sun, 30’.

Slide 8

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 9

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.

- Astronomy and Space Science
- Length: Power of Ten
- Units of Length
- Time Scales
- Angles
- Celestial Sphere
- Useful Relations
- Motion of the Sun on Celestial Sphere
- Ecliptic Plane
- Planetary Motion on Celestial Sphere
- Advantages of the Heliocentric Model
- Further Developments
- What is a constellation?
- How does the coordinate systems on the Celestial sphere look like?
- Where exactly is the center of the celestial sphere?
- I heard that the definition of the ecliptic plane has been changed, is it?
- What’s the relation between solar motion and the calendar?
- How was the Sun/Earth orbit modeled by Greek astronomers?
- What is the cause for precession of the equinoxes?
- What is a day anyway?
- Can you give some example of planetary events?
- What is Aristotle’s model of the universe?
- What does Ptolemy’s geocentric model look like?
- Ptolemy model looks quite different from Kepler’s, why did it work so well?
- How to transform between geocentric and heliocentric models?
- One arcmin is about the size of a HK$1 coin in 88 m away, how did Tycho Brahe achieve this accuracy without telescopes?
- Did Galileo really invented the telescope?
- Was Galileo jailed?
- Does the discovery of phase of Venus disproves the geocentric theory?
- What is a planet?
- After the invention of telescope, how was position/angle measured?
- We’ve been focusing on the development of the West, what about the work of Chinese?
- Was Copernicus the first to think the Earth moves around the Sun? Did Copernicus model have epicycles?
- What are the true advantages of the heliocentric model?
- What is the role of human/Earth in cosmology?
- Can you suggest some equipments for schools?
- Can you give us some references?
- Are there any useful classroom teaching kits available?
- Sources of Pictures in this Talk

- Radioactivity and Nuclear Reactions
- Newton’s third law of motion
- Newton's Laws
- Solar Thermal Energy
- Sound
- Waves & Sound
- Static and Kinetic Friction

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