Time Flashcards
What is the apparent and mean sun?
The apparent sun is the actual sun we can see. The mean sun is the imaginary sun which moves with constant speed across the sky equal to the mean rate of the real sun.
What is the actual rotation period of the Earth?
23 hours 56 minutes
What time does it take for the Earth to rotate through 1°?
4 minutes
What is a solar day?
The time it takes for the apparent Sun to cross the observers meridian consecutively.
What is a sidereal day?
The time it takes a star to cross the observers meridian consecutively (23h 56m (04s)).
Why is the solar day longer than the sidereal day?
Because of the Earth’s revolution - it orbits the Sun at the same time as rotating on its axis and therefore has a rotational period shorter than 24 hours.
What is the equation of time?
The apparent solar time - the mean solar time
What is universal time and what is it useful for?
GMT for the whole world - useful for keeping time when off the Earth.
How can you use a sundial to tell the time?
Gnomon on the sundial casts a shadow which points at a specific time.
How can you use a shadow stick to determine local noon?
When the shadow is the shortest it is local noon - plot a graph of shadow length against time (GMT) and look at shortest length.
How can you use a shadow stick to find your latitude?
Elevation of sun = tan^-1 (height of stick/shadow length)
Observers latitude = (90° - elevation of sun) + declination of sun
