4. Time and the Earth, Moon, Sun System

Question 1

How is the position of the Sun used to determine Apparent Solar Time? (AST)

Answer 1

AST is the time taken on a sundial which is based on the position of the sun in the sky as seen from Earth

Question 2

How is the Mean Sun used to determine Mean Solar Time? (MST)

Answer 2

• The Mean Sun is the position of the sun based on a circular orbit instead of the elliptical one
• This lets us used a fixed length of day and mechanical methods to determine the time
• MST is measured on a clock/digital device and uses UT/GMT

Question 3

How is Local Mean Time found? (LMT)

Answer 3

LMT is the mean solar time for a specific location on Earth

Question 4

What is the Equation of Time?

Answer 4

EOT = AST - MST

Question 5

Why does the EOT vary throughout the year?

Answer 5

eccentricity (measure of elliptical orbit) - affects synodic days, earths orbit slightly elliptical. when at perihelian, moves faster, travels further, makes synodic days longer
obliquity of elliptic (tilt of earths axis) - affects synodic days, makes sun change declination. further from equator = longer days
these affect the MST, affecing the equation

Question 6

How do you define GMT?

Answer 6

Mean time at any point along the Prime Meridian (vertical line) is defined as Greenwich Mean Time, which is the same as Universal Time (UT)

Question 7

How do you analyse shadow stick data to find local noon?

Answer 7

• Draw a line of best fit on the graph to find out the lowest point of the sun
• Wherever the shadows are the shortest, that is local noon

Question 8

How do you use shadow stick data and the EOT to find longitude of the observer?

Answer 8

Use the formula AST = EOT - MST
The data from the shadow sticks will give you the time of Local Noon/Mean Solar Time
The Equation of time will be given as a number (eg -6, 8)
Greenwich Mean Time will always be 12:00, take away the EOT from this
Find the difference between the AST and the GMT-EOT time
Divide this difference by 4 - that is the longitude (then add E or W)

Question 9

Why is local time different in different longitudes?

Answer 9

If you move 1 degree either way, your midday will be 4 minutes earlier/later than your previous location

Question 10

How do you read the time from a sundial?

Answer 10

• Sundials use the light from the sun to create a shadow that falls on the dial - used to tell the time
• 12’o’clock & the gnomon (part that makes the shadow) should be facing north

Question 11

Explain the Lunar Phase cycle

Question 12

What is the relationship between sidereal and synodic/solar time?

Answer 12

The sidereal day is 4 minutes shorter than the solar day

Question 13

What is the difference between sidereal and synodic/solar days & months?

Answer 13

Sidereal Day - 23h 56m
Sidereal Month - 27.3d
Synodic Day - 24h
Synodic Month - 29.5d

Question 14

Why does the time of sunrise and sunset vary throughout the year?

Answer 14

• Earth has an elliptical orbit
• Earth’s rotation axis isn’t flat with the plane of the orbit
• This results in uneven changes in the times of sunrise/sunsets

Question 15

What is the significance of equinoxes and solstices?

Answer 15

They determine the changing of seasons

Question 16

Why do we need time zones?

Answer 16

• When trains were introduced, people found themselves losing/gaining time & would have to reset their watch
• Messages would also be sent at incorrect times
• Time zones were made to help this problem, and create a unified time keeping system
• Different parts of the Earth enter/exit daylight at different times

Question 17

What is meant by ‘culminating’?

Answer 17

When the sun is at the highest point of the sky