Topic 1: Naked eye astronomyTopic 1: Naked eye astronomy


Topic 1: Naked eye astronomy

Astrolabe – measures the angle of a star above the horizon. Used for navigation.

Eclipse – the sun moves behind the moon (allows you to observe gasses in the corona)

The sun rises at the east and sets in the west, at a steady speed.

Stars rotate in the sky, directly above the Earth’s poles.

What can we see in the sky? 1A

People thought Earth was centre of universe b/c sun, moon, and stars seemed to revolve around it.

But the Earth is tilted – spins on its axis. It is the Earth that spins, NOT the stars.


Moon reappears around Earth every 25hrs.

Moon takes 28 days to make an orbit the Earth

Earth travels around sun every 24hrs                                                 

Earth orbits Earth in 365 days.

Sun = East to West                          Moon = West to East

Phases of the moon

The moon reflects light.

The half of the moon facing the sun is lit up

  • Reflects light to Earth thus it is daylight).

The half turned away from the moon is in shadows

  • Cannot reflect light thus it is night).

Full moon = lit up side is wholly facing the Earth

New moon = shadows side is wholly facing the Earth.

Mapping the sky: constellations

Constellation – a group of stars

Superstition – Astrologers use movement of planets across constellations to tell fortunes

Seasonal skies

Earth travels halfway around its orbit in 6 months. This is when you see the opposite side of the sky.

A star seen from Earth has actually raised 4mins earlier.

6 months x 4mins = 12 hrs à half a day

Therefore, a star that rises at dusk in June will set at dusk in December.

Stars are at a fixed backdrop, whereas planets move.


Retrograde Motion 1B

Planets orbit the sun at different speeds but in the same direction.

Planets further = wider orbits = slowest à more days to orbit

Planets closer = smaller orbits = fastest à fewer days to orbit

With the naked eye, at dawn you can see some planets (e.g. mercury, Jupiter) at dawn / dusk because they move down towards Earth during their orbit. To track the planets you should note down how long it takes for the planet to reach its position (sidereal). The planets will be moving from West to East.

Retrograde motion – when the planets seem to move in the opposite direction

  • Seem to travel East to West rather than their usual West to East
  • This only happens to planets outside the Earth’s orbit, NOT Mercury / Venus
  • This cycle repeats every approx. 2 years


Diagram of Retrograde Motion

Interpreting graphs

  • Travel east àA increases
  • Travel west àA decreases
  • The travels east again àA increases







Eclipses 1C

Angular size – depends on object size + distance away.

Solar eclipse – the moon is at the right size and distance to block the sun’s light

  • Angular size of moon – 0.5 degrees

Some parts of Earth have total solar eclipse and are pulled into umbra (total darkness).

Other parts have a partial solar eclipse and are pulled into penumbra (partial darkness).

Lunar eclipse – Moon passes into Earth’s shadow, which is bigger than the moon’s shadow.

  • Earth prevents sunlight from reaching and reflecting off of the moon

Partial lunar eclipse – when the moon isn’t fully in the earth’s shadow

  • Some light is reflected so not all dark in the areas.

Lunar = Earth blocks moon

Solar = Moon blocks sun








Why the rarity of eclipses?

The moon orbits the Earth at an angle (5 degrees) to the Earth’s orbit of the sun.

Total eclipses only occur when the moon and earth + moon and sun line up perfectly.

Partial eclipses happen more often b/c they don’t line up properly.



The moon’s orbit around the Earth is longer than the Earth’s orbit around the sun.

It takes 25hrs to appear in the same position as it travels from West to east (just like the planets). It takes 25hrs b/c the moon is orbiting the Earth at the same time that it is spinning on its axis. They cannot be properly aligned unless the moon has extra time to orbit the Earth.

One complete moon orbit around the Earth takes 28 days.


Side real day


Sidereal day – time taken for star to return to the same position in the sky

  • The Earth has to rotate 360 degrees for this to happen.
  • One full rotation happens every 23hrs 56mins.



Solar day

Solar day – time taken for the sun to appear at the same position in the sky. It takes 24hrs.

The reason why the sun takes 4mins longer is b/c the Earth is spinning on its axis as well as rotating around the sun. Because the Earth is rotating, its angle towards the sun has slightly changed. Although the Earth has rotated 360 degrees and the stars can be seen in the same location, the sun is not directly overhead yet. Thus, the sun needs to rotate slightly more than 360 degrees (4mins worth) so that it can be directly overhead again.

Same patch of sky

The Earth is rotating in a circular orbit. This means that the direction we are facing at slightly changes as the Earth rotates. It takes a year for the Earth to make a complete orbit of the sun. On the same day after one year you can see the same patch of sky and stars.

Celestial Sphere

Pole star is in a fixed position b/cit is directly above the North Pole.

  • You can use this to measure the declination (degrees) which is the celestial latitude (vertical).


The celestial equator is an imaginary plane across the sky.

  • You can use this to measure right ascension (degrees or time) which is the celestial longitude (horizontal).
  • Right ascension increases the further east you go





Astronomers use these measurements to give precise positions b/c the Earth rotates, so the position of the stars in the sky every 24hrs and look like they’re in different directions.


There is a reference point in the sky, also known as vernal equinox.



Using right ascension and declination


You can use these measurements to locate where the star is.



Declination line + R.A line = missing side

You need to find the missing side to find the missing angle.