P7: Naked Eye Astronomy

P7: Naked Eye Astronomy

1. The Earth Rotates on its axis

   • It does this West to East once in just under 24 hours
     • This causes the stars to appear to move East to West in the same amount of time
     • The sun, planets and moon also appear to move east to west across the sky, however the time they take to do this depends on their individual orbits
       • The sun appears to travel across the sky once every 24 hrs
   • A sidereal day is the time it takes for the earth to rotate 360 degrees on its axis
   • A solar day is the time from noon one day, to noon the next (i.e. 24 hrs)
     • The earth orbits the sun as well as rotating on its axis
       • It is this that makes the sidereal day 4 minutes shorter that a solar day
       • Different stars can be seen at different times of the year, depending on the position of the earth in the suns orbit

2. The Position of Astronomical Objects

   • The position of astronomical objects can be measured in terms of angles as seen from the earth.
     • Angles of "Declination" and "Right Ascension" describe the positions of stars in relation to a fixed point on the equator
       • Declination is measured south and north of the Equator
         • A star with Positive declination will be visible form the northern hemisphere
         • A star with Negative Declination will be visible from the southern hemisphere
       • Right ascension is measured around the celestial equator
   • Planets can be seen from earth with the naked eye, they look similar to stars but change their positions in a complicated pattern
     • When compared to backgrounds of fixed stars, planets appear ti move with a retrograde motion
       • This is because the earths smaller orbit causes it to overtake each of the outer planets at different times of the year.
         • This causes these planets to slow down against the star background and go into reverse
       • Retrograde motion- The apparent backwards motion of planets from west to east due to the combined motion of earth and the planets

3. The Moon

   • Whilst the Earth is rotating on its axis, the moon is orbiting the Earth in the same direction
     • Due to this, the moon appears to be moving East to West across the sky in a little over 24 hours, faster than the earth
   • The Lunar Cycle describes the moons appearance during its 28 day orbit
     • The moons shape during its orbit is due to the part of the moon that is visible to the earth
       • We are able to see the moon because of the suns light reflecting off it
         • The side facing away from the sun appears dark
         • The side facing towards the sun appears light

4. Eclipses

   • Solar Eclipse
     • This occurs when the moon passes between the earth and the sun
       • This results in the moon casting a shadow on the earth
     • Total Solar Eclipse
       • Occurs when the moon is directly in front of the sun and completely obscures the earths view of the sun
   • Lunar Eclipse
     • Occurs when the earth is between the sun and the moon
       • Results in the Earth casting a shadow on the moon
   • Lunar eclipses don't occur every month because the moon orbits the earth at a 5 degree angle to that of the earths orbit of the sun
     • An eclipse can only occur when the moon passes through the ecliptic ( the apparent path the sunlight traces out along the sky)
       • There are between 2 and 5 solar eclipses ever year
       • A total eclipse will only occur roughly ever 18 months