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Time Flashcards

1
Q

Are SR/SS times and twilight durations the same at all longitudes on a given latitude?

A

In practice, yes.

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2
Q

How to calculate UTC given LMT?

A

(1) Divide longitude by 15
(2) Add/subtract the result from (1) to/from LMT

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3
Q

Define: sunrise.

A

The instant where the upper limb of the Sun appears over the horizon.

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4
Q

Define: astronomical twilight.

A

The length of time when the geometric centre of the Sun is between -18° and -12° below the horizon.

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5
Q

What does //// signify in the Air Almanac?

A

Continuous civil twilight.

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6
Q

What times are given in the three lists of the Air Almanac?

A

UTC (LMT at the GM).

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7
Q

Of the following, which is the shortest:

(a) Sidereal day

(b) Apparent solar day

(c) Mean solar day

A

Sidereal day, because it depends only on the Earth’s rotation relative to stars. The Earth’s elliptical orbit and its axial tilt are not considered.

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8
Q

Define: sidereal time.

A

Time based on the Earth’s orientation in space.

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9
Q

How do we calculate Δt from Δλ?

A

By applying the standard conversion factors:

360° = 24 h

15° = 1 h

1° = 4 min

15’ = 1 min

In practice, dividing Δλ by 15 gives the Δt.

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10
Q

Define: the transit of the Sun.

A

The sun transits when it reaches directly overhead an oberver’s meridian. At this time, the Sun is at its highest point in the sky (from the observer’s perspective).

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11
Q

How to calculate SS at a position B from SS at a position A (all times in UTC)?

A

(1) Calculate Δλ
(2) From Δλ infer the time difference (15° = 1 h)
(3) Apply the time difference, adding or subtracting as necessary (+ W, - E)

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12
Q

Why SR and SS times vary throughout the year?

A

Because of the Earth’s axial tilt and its orbital position.

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13
Q

Define: Local Mean Time.

A

Time measured with reference to the mean Sun.

LMT is different at each meridian of longitude.

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14
Q

How long is one mean solar day?

A

24 h.

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15
Q

Is it correct to say that, in general:

UTC = GMT = Greenwich LMT = ZULU

?

A

Yes. There are differences but in real, day-to-day practical life these will be negligible.

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16
Q

What does arc to time relationship signify?

A

The ability to calculate the difference in local time between two positions with Δλ known.

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17
Q

How does date change when crossing from one STZ into another STZ if one STZ is past midnight and the other is still not?

A

Crossing west to east: calendar is advanced one day

Crossing east to west: calendar is reversed one day

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18
Q

What is another common name for:

(a) civil dawn

(b) civil dusk

?

A

(a) morning civil twilight
(b) evening civil twilight

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19
Q

In which direction does the Earth rotate?

A

Viewed from above the NP, anticlockwise.

Viewed from above the equator, from W to E.

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20
Q

Are GMT and UTC the same?

A

No.

GMT is based on the mean sun at Greenwich.

UTC is based on an atomic clock.

However, in practice they are close enough to be called the same.

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21
Q

Define: sunset.

A

The instant where the upper limb of the Sun disappears below the horizon.

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22
Q

How to calculate time at location B from time at location A?

A

(1) Find ΔΦ
(2) Divide the result from (1) by 15
(3) Add or subtract the result from (2) to/from the time at A

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23
Q

How long is one sidereal day?

A

23 h, 56 min, 4 s.

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24
Q

Define: sidereal day.

A

Time taken for one Earth’s rotation with reference to a fixed point in space.

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25
Q

In the Air Almanac, SR/SS times are usually given in a tabular form for every couple of degrees of latitude. How can we know SR/SS times for a latitude not explicitly given in the Air Almanac?

A

By linear interpolation, which in practice yields very precise results.

26
Q

What is standard time?

A

The practical solution to timekeeping. In a standard time zone, the LMT of the closest meridian is used (for convenience), even though that time might not represent the true LMT in the whole area.

27
Q

Define: civil twilight.

A

The length of time when the geometric centre of the Sun is between -6° below the horizon and the horizon.

Civil twilight is supposed to be a period in which an ambient light level is bright enough to allow activities to be carried out without the need for artificial light.

28
Q

What is Analemma?

A

Graph of the equation of the time. It has a figure of the number 8. It signifies the changing angular offset of the Sun from its mean position on the celestial sphere over the course of one year.

29
Q

What do axes represent on the equation of time graph?

A

Vertical axis represents the Sun’s declination (-23.5° to 23.5°) while the horizontal axis represents time (how much the apparent Sun is ahead or behind the mean Sun).

30
Q

The duration of an apparent solar day is not constant. It varies throughout the year. What factors are mostly responsible for that?

A

Different orbital speeds of the Earth (faster at perihelion than at aphelion) and the Earth’s tilt (angled apparent motion of the sun at equinoxes).

31
Q

Define: twilight.

A

Transitional period from night to day or vice versa when there might be sunlight illumination even if the Sun is not visible.

32
Q

Define: Greenwich Mean Time.

A

LMT at Greenwich, London.

33
Q

What time is the international aviation standard?

A

Universal Coordinated Time. At a given time, UTC is the same at all locations.

34
Q

How can we calculate the duration of:

(a) MCT

(b) ECT

?

A

(a) SR - MCT start
(b) ECT end - SS

35
Q

How does longitude of a position affect SR and SS times of that position?

A

In practical terms, longitude has no effect on SR/SS times.

36
Q

Where on the Earth is the variance of daytime length the smallest?

A

At the equator.

37
Q

Explain why are sidereal and apparent solar days of different lengths.

A

The sidereal day depends only on the Earth’s rotation (reference = fixed point in space) whereas the apparent solar day also depends on the Earth’s movement around the Sun and its tilt.

38
Q

Define: ZULU time.

A

It’s the same as GMT time (and therefore also the same as UTC time). Since GMT is based on the location of the zero (Greenwich) meridian, GMT times are also called Z (from zero) times. ZULU times (GMT times) have a ‘z’ next to them.

39
Q

Define: mean solar day.

A

Average length of the apparent solar day.

40
Q

How does local time change when travelling?

A

Travelling east = time increases

Travelling west = time decreases

41
Q

What does the equation of the time signify?

A

Mathematical relationship between MST and AST.

42
Q

What do white rectangles signify in the Air Almanac?

A

That the Sun does not set.

WHITE = LIGHT

43
Q

What’s the Air Almanac for?

A

It’s used for:

» conversions between ST and UTC

» calculations of SR, SS, MCT, and ECT times at different locations

44
Q

What significance does the change of longitude have in ST calculations?

A

None. Standard time calculations are performed with the Air Almanac and UTC.

45
Q

How many lists does the Air Almanac have and what do these lists represent?

A

It contains three lists:

(1) Places where ST is ahead the UTC (east of Greenwich)
(2) Places where ST = UTC
(3) Places where ST is behind the UTC (west of Greenwich)

46
Q

How to calculate ST from UTC?

A

From the Air Almanac, which indicates the time difference between the two for a given location.

47
Q

What is the maximum difference in UTC time that can exist between two locations?

A

There cannot be any difference in UTC time. UTC time is standard time and is the same everywhere.

48
Q

Is the midday local Sun always at the same celestial location?

A

No, it varies in accordance with the graph of the time equation (Analemma).

49
Q

In what sense does time change with regard to DST?

A

SPRING FORWARD - FALL BACK

50
Q

What is Daylight Saving Time?

A

Adjustment of time done biannually (in Spring and Autumn) to account for the increase/decrease of daylight duration.

51
Q

At which latitudes does twilight last:

(a) the shortest

(b) the longest

(c) and why?

A

(a) in equatorial regions
(b) at higher latitudes
(c) because of the way the Sun transits the equator (higher latitude = shallower ascent = more time to achieve the same ascent as that of lower)

52
Q

What’s the difference between Daylight Saving Time and Summer Time?

A

They both mean the same thing. Sometimes the word Summer is added to a standard time zone name as an indication of it using DST.

53
Q

In flight, the calendar date may change. In which two situations will that occur?

A

(1) Crossing the IDL
(2) Crossing the border between two ST zones where the time is after midnight in one zone but before midnight in the other zone or vice versa

54
Q

Define: nautical twilight.

A

The length of time when the geometric centre of the Sun is between -12° and -6° below the horizon.

55
Q

When are days and nights equally long?

A

At March and September equinoxes.

56
Q

How to calculate ST at a position B from ST at a position A?

A

(1) Refer to the air almanac and note ST for A and B
(2) Calculate the total difference in ST between A and B
(3) Add/subtract the difference in ST to/from A

57
Q

What do black rectangles signify in the Air Almanac?

A

That the Sun does not rise.

BLACK = DARK

58
Q

Define: apparent solar day.

A

Time taken between two transits of the Sun.

59
Q

Regarding time calculations, when do we add times and when do we subtract them?

A

West is behind. Towards west: subtract.

East is ahead. Towards east: add.

60
Q

What is the purpose of the equation of time graph?

A

To show the difference in time between the midday apparent Sun and the mean Sun over the course of one year.

61
Q

When is the difference between AST and MST the greatest and what are the related time values?

A

The maximum difference between the two occurs in:

(1) February, with the apparent sun 14.5 min behind the mean sun
(2) November, with the apparent sun 16.5 min ahead the mean sun

62
Q

What times are referenced in SR/SS and twilight tables?

A

LMT.

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