Time and Longitude

Question 1

Time Zones and Relation to Longitude

Answer 1

● The “Mean Solar Day” is the interval between two successive passes of the sun over a given meridian of longitude.
● Since the sun appears to travel a complete circle around the earth everyday it can be said to traverse 360° of longitude per day.
● As a result we can express mean time in terms of longitude, and longitude in terms of mean time

● For example
➢ 24 hours = 360º longitude
➢ 1 hour = 15º longitude
➢ 1 minute = 15‘ longitude
➢ 1 second = 15“ longitude

Question 2

Local Mean Time

Answer 2

● Local Mean Time or LMT is the specific time at a particular meridian.
● The LMT of a particular meridian will differ from every other meridian by between 0 and 24 hours.
● Due to the fact that EVERY meridian has a different Local Mean Time, Time Zones were created.
● Since the earth rotates through 15° of longitude every hour this was defined as the “width” of a time zone.

Question 3

Twilight

Answer 3

● Twilight is the time between day and night when there is light outside but the Sun is below the horizon.
● Twilight occurs when the Earth’s upper atmosphere scatters and reflects sunlight and illuminates the lower atmosphere.
● Astronomers define them on the basis of the Sun’s elevation, the angle that the geometric center of the Sun makes with the horizon

According to CARs 101.01, Night means the time between the end of evening civil twilight and the beginning of morning civil twilight. Day means the time between the beginning of morning civil twilight and the end of evening civil twilight.

Question 4

Civil Twilight

Answer 4

● In the morning, civil twilight begins when the centre of the sun’s disc is 6° below the horizon and is ascending, and ends at sunrise approximately 25 min later.
● In the evening, civil twilight begins at sunset, and ends when the centre of the sun’s disc is 6° below the horizon and is descending approximately 25 min later.
● Civil twilight is the brightest form of twilight.
○ There is enough natural sunlight during this period that artificial light may not be required to carry out outdoor activities.
○ Only the brightest celestial objects can be observed by the naked eye during this time
● The length of twilight depends on latitude.
○ Equatorial and tropical regions tend to have shorter lengths of twilight than locations at higher latitudes.

Question 5

practice

Answer 5

notes