Topic 4- Time And Earth Moon Sun Cycles Flashcards
What is the equation for equation of time
EOT = AST - MST
How many minutes does 1 degree equal
4 minutes
What is AST
is apparent solar time it is the time the sun shows in the day we can use a sundial for this
What is MST
Is mean solar time it is the our watches that humans have invented as the sun is an unreliable way to tell time in society
How many minutes can AST be inaccurate
16 minutes
What cause AST to be inaccurate
Caused by earth elliptical orbit and axis of earth
How many times a year is a sundial accurate
4 times
How do you determine the angle of a gnomon on a sundial
The latitude of place
Where does gnomon on sundial point to
NCP
What are the disadvantage of sundial
Useless when no sun
Inaccurate due to the EOT
What day does the summer solstice happen on
21st of june
Where does the sun set and rise on the summer solstice
Sun sets in north west and sun rises in north east
When is the winter solstice
21st December
Where does the sun rise and set on the winter solstice
Sun rises in the south east and sets in the south west
When do the equinoxes happen
21st of March and September
Where does the sun rise and set on the equinoxes
Rises in east set in west
What is a solar/synodic day
Is the time it takes the sun to arrive in the same position as the day before
What is a sidereal day
When we measure the rotation not by the sun but by the stars
How long is a solar/synodic day
24 hours
How long is a sidereal day
23 hours 56 minutes
What method do you use to find your local noon
Shadow stick
If you moon one degree either way of your location the sun will still culminate at midday but your middy will be when
Will be four minutes earlier or late than previous location
What made people convert to MST and why
Trains when they were invented people would fin themselves losing or gaining time when they had reached their destination
When more countries started converting to MST what did they decide
They needed a longitude starting point
How many meridians did the earth get split into and how many degrees apart
24 meridians and 15 degrees apart
Who recognised the problem with with using pendulum clocks on ships
John Harrison
What did the horological method help with
Navigators could now tell the local time at Greenwich and easily calculate the difference between there local time and they no longer needed to do complex observations or rely on good weather
What was used prior to the introduction of the chronometer
Lunar distance
What does lunar distance involve
Calculating the distance between the moon and a prominent star to find navigators longitude
How many degrees does the moon move across the sky in 24 hours
13 degrees
How did they use lunar distance to calculate longitude
A navigator would measure the the altitude of a prominent star above the horizon
Then they would observe the angle between the moon and star
They then would consult an almanac containing tables of data based on Greenwich
Work out their longitude by using the difference
What are the flaws of the lunar distance method
You need a clear sky
Accurate measure to other wise it could result in the navigator being several nautical miles away from destination
Earth is not completely spherical
