9. TIME

INTRODUCTION

WHAT’S TIME?HOW CAN WE MESURE IT?

WHERE ARE WE? WHEN ARE WE?

INTRODUCTION

THE SOLAR SYSTEM

THE SUN9 MAJOR PLANETSPLANETARY MOONS2000 MINOR PLANETS & ATEROIDS

INTRODUCTION

INTRODUCTION

INTRODUCTION

INTRODUCTION

INTRODUCTIONKEPPLER LAWS:

1. Each planet moves in an elliptical orbit with the Sun in one foci

INTRODUCTION2. The straight line joining the Sun and any planet

sweeps out equal areas in equal time intervals

INTRODUCTION3. K . SP² = d³

INTRODUCTIONRELATIVE POSITION EARTH – SUN:

• PERIHELION Closest to the Sun• APHELION Furtherst from the Sun

INTRODUCTION

EARTH’S MOVEMENTS:1. ROTATION (West -> East) DIURNAL CHANGES2. ANNUAL MOVEMENT (Orbit) Due also to axis

inclination in relation to the Sun’s ecliptic. SEASONAL CHANGESElliptic movement arround Sun (one foci)

IMPORTANT POSITIONS (N.H):SUMMER SOLSTICE 21st JuneWINTER SOLSTICE 22nd DecVERNAL or SPRING EQUINOX 21st MarchAUTUMNAL EQUINOX 23rd Sep

INTRODUCTION

INTRODUCTION

INTRODUCTION

INTRODUCTION

SEASONS OF THE YEARIMPORTANT FEATURES:

• Earth axis tilted 66.5º to the plane of the ECLIPTIC. ANGLE BETWEEN EQ AND ECLIPTIC = 23.5º

Paralel of latitude directly underneath the Sun rays changes slowly

SEASONAL CHANGES OVER THE YEAR (DECLINATION)

SEASONS OF THE YEARIMPORTANT FEATURES:

DECLINATION

SEASON OF THE YEAR

Sun at:

• Most SOUTHERLY point (Tropic of Capricorn): 22nd Dec

• Most NORTHERLY point (Tropic of Capricorn): 21st June

N.H WINTER SOLSTICES.H. SUMMER SOLSTICE

N.H. SUMMER SOLSTICES.H. WINTER SOLSTICE

SEASON OF THE YEARSun crosses the Equator:

• From South to North: 21st of March

• From North to South: 23rd of September

N.H. SPRING or VERNAL EQUINOXS.H. AUTUMN EQUINOX

N.H. AUTUMN EQUINOXS.H. SPRING or VERNAL EQUINOX

THE DAYDEFINITION: The time interval that elapses between two

succesive transits of a heavenly body across the same meridian

Earth rotation ≈ Rotation of celestial sphere

Heavenly bodies are continuosly crossing an observer’s meridian in an East – West direction

THE DAYAny heavenly body could be used as a timekeeper

Sun not the perfect one because its apparent speed along the ecliptic varies

SIDEREAL DAY: Measured against any star (Unable to relate it with light darkness periods)SOLAR DAY: Measured against the Sun

THE DAY

THE DAYAPPARENT SOLAR DAY:

Time interval that elapses between two successive transits of the actual Sun across the same meridian

APPARENT SOLAR TIME: Time based upon the Sun as it appears on the sky, taking as reference its transit from the observers antimeridian.

Measured in refrence to the apparent Sun on a particular meridian.

THE DAY

The Sun’s real movement on the ecliptic per day is more than 360º. There are two reasons:

• The Earth is moving on an ecliptic motion arround the Sun

• The Earth’s speed is varying arround its orbit

THE DAYTHE MEAN SOLAR DAY Due to the fact of the Sun not moving equally on the sky every day , and also to the fact of this variation not being constant, we can’t consider the Real Sun giving us a practical unit of measurement

To overcome this dificulty IMAGINARY BODY = MEAN SUN.Its assumed to move along the celestial equator (ecliptic) at a uniform speed, and to complete one revolution in nearly the mean time it takes to the true Sun to

complete one revolution in the ecliptic

THE DAYTHE MEAN SOLAR DAY

DEFINITION: Time interval between two succesive transits of the mean Sun across the same meridian. CIVIL DAY

360º of longitude = 24 Mean solar hours15º of longitude = 1 Mean solar hour

NOTE: Difference between Apparent and Mean days is nearly a minute

THE DAYEQUATION OF TIMEDEFINITION: Relation in terms of time between the Apparent

time and Mean time for a specific day. Varies throughout the year and its due to the eccentricy* of the Earth’s elliptical orbit arround the Sun

EQUATION OF TIME = APPARENT TIME – MEAN TIME

ECCENTRICITY = Relationship between the major and minor axes of an ellipse. Meassures how simmilar an ellipse and a circumference are

THE DAYEQUATION OF TIME

THE YEAR

SIDERAL YEAR: Time the Earth takes to complete a full orbit arround the Sun measured against a distant star . 365d 5h 48min 45 sec. For ease, 365days and 6hours

CALENDAR YEAR: Taken as 365days. Kept in accordance with the calendar year by adding 1 day to the year each 4 years (Leap year)

LOCAL MEAN TIME (LMT)

DEFINITION: Time according to the Mean Sun.The angle, converted to time, from the observers

antimeridian westwards to the Mean Sun

Diff Long (A-B) Diff of LMT (A-B)

NOTE: A 24h period implies a 360º rotation and, because of this, 15º rotation (CHLong) is a 1h change in time Conversion Arc to time Chart

LOCAL MEAN TIME (LMT)

LOCAL MEAN TIME (LMT)UNIVERSAL CO-ORDINATED TIME (UTC/GMT)

LMT at the Greenwich meridian (0ºE/W). Is the time reference for aviation.

TIME CONVERSION CALCULUS BETWEEN ANY LMT AND UTC

Where a meridian is:• East of Greenwich Time is LATER• West of Greenwich Time is EARLIER

Long WEST, UTC BESTLong EAST, UTC LEAST

LOCAL MEAN TIME (LMT)DIFFERENCE BETWEEN UTC AND GMT:

Co-ordinated Universtal Time (UTC) changes at a constant rate and is regulated agains the INTERNATIONAL ATOMIC TIME (IAT), which is derived from atomic clocks. UTC is regularly corrected to match GMT (LMT at the Greenwich Meridian), but those corrections are very small.

For practical purposes, UTC equals GMT

LOCAL MEAN TIME (LMT)UNIVERSAL CO-ORDINATED TIME (UTC/GMT):

Example 1: What’s the difference in LMT between London Heathrow (51º 28’N 000º27’W) and Kennedy Intl (New York) (40º38’N 073º46’W)

1. CHLong = 73º46’ – 000º27’ = 73º 19’2. Knowing that 15º change in longitude equal 1h change in time:

73º19’ = 73.316º 73.316º · (1h/15º) = 4.887h = 4h 53min

LOCAL MEAN TIME (LMT)UNIVERSAL CO-ORDINATED TIME (UTC):

Example 2: If the LMT in Goose Bay (060ºW) is 1200, what is the UTC?

1. CHLong = 060ºw – 000ºE/W = 60º2. 60º · (1h/15º) = 4.0h3. As Goose Bay is West (Longitude west UTC best) we have to add this

difference in LMT to the time of Goose Bay to find the UTC

1200 + 4 = 1600

ZONE TIME

DEFINITION:Earth is divided into 24 time zones, each of 15º of

longitude in width.Each zone has 1h difference to the one right next to it and uses the LMT of its central meridian as time

reference.

ZONE TIME

ZONE TIMEFEATURES:

• Each zone is designated by letters: zone 0 = Z, zone -1 = A,…

• Eastern longitudes are numbered with negative zone numbers

• Western longitudes are numbered with positive zone numbers

• Mathematically UTC = Zone Time + Zone number

ZONE TIME

STANDARD TIMEDEFINITION:

Time stated for a determined area in accordance with State’s frontiers of natural geographical borders.

Sometimes listed as LOCAL TIME (LT)

Countries like USA, Canada or Australia have such a large east-west extent that need to use multiple time belts, each of them having its own Standard Time

STANDARD TIME

STANDARD TIMEDIFFERENCE BETWEEN STANDARD AND ZONE TIME

STANDARD TIMESTANDARD TIME TABLES (Book):

LIST 1: Contains places where ST is normally fast on UTC (East of Greenwich Meridian)

LIST 2:Contains places where ST is normally UTC/GMT

LIST 3: Contains places where ST is normally slow on UTC (West of Greenwich Meridian)

STANDARD TIME INTERNATIONAL DATE LINE:

Whenever taking into account LMT, and moving westwards there will be a moment in which we will get to the Greenwich anti-meridian. There its supposed to be a -12h LMT.

If we do the same eastwards, will get to the same point, but with a supposed time of +12h.

Whenever we cross the 180ºE/W meridian we will be, then, changing day, and depending on which direction we are crossing it to, we will have to change date foreward or backwards

STANDARD TIME INTERNATIONAL DATE LINE:

STANDARD TIME INTERNATIONAL DATE LINE:

STANDARD TIME

STANDARD TIME

STANDARD TIME

STANDARD TIME

DAYLIGHT SAVING TIMEEXPLANATION:

Most countries adjust their local time (ST) according to the season of the year. When this is done, local time is advanced one hour in the spring, and the Daylight Saving Time (Also called Summer Time) is in efecct. In autumn the clocks are set back to ST again Energy saving

Countries following this rule will be specifyied on the ST Lists by and asterisk *

TIME EXERCICES

TIME EXERCICES

TIME EXERCICES

TIME EXERCICES

TIME EXERCICES

TIME EXERCICES

SUNRISE & SUNSETSUNRISE: Time moment of the day in which the upper limb of

the Sun is coincident with the observer’s visible horizon, and the Sun’s tendency is to climb up on the sky.

Centre of the Sun in 0.8º bellow the horizon but due to refraction the Sun’s visible before those 0.8º

SUNRISE & SUNSETSUNSET: Time moment of the day in which the upper limb of

the Sun is coincident with the observer’s visible horizon, and the Sun’s tendency is to fall bellow the horizon.

Centre of the Sun in 0.8º bellow the horizon

SUNRISE & SUNSET

DEPENDENT ON:

1. DATE:

• SUMMER SR earlier and SS later• WINTER SR later and SS earlier

Times for SR & SS change slightly from one year to another

SUNRISE & SUNSETDEPENDENT ON:

2. LATITUDE:

Times for SR and SS change by several min each 3-day period and this change is more pronounced the higher the atitude.

For one particular latitude, all places, regardless of longitude, will have the same LMT for SR and SS

TWILIGHT

DEFINITION:

Period of the day before sunrise and after sunset in which there is still sifficient illumination for normal daylight operations to continue.

TWILIGHTPeriod divided into 3 stages:

CIVIL TWILIGHT – Time period between the moment of the Sun being 6º bellow horizon until SR. Or betwee SS and 6º bellow horizon. SETS LIMIT FOR DAY-FLYING

NAUTICAL TWILIGHT - Time period betwee the 12º-bellow-horizon position of the Sun until the Civil twilight begins

ASTRONOMICAL TWILIGHT – Time period between the moment in which the Sun is 18º bellow horizon to the moment the Nautical twilight begins

TWILIGHT

TWILIGHT1. VARIATION OF TWILIGHT WITH LATITUDE:

LOW LATITUDES: At the Tropics the Sun rises and sets at almost 90º to the horizon. The period is quite short because the way traveled is the shortest possible

HIGH LATITUDES: The angle of the Sun approaching the horizon is tilted, the path followed until those 6º bellow horizon will, then, be longer and so will be the time taken.

Lasts more at high latitudes

TWILIGHT1. VARIATION OF TWILIGHT WITH LATITUDE:

LOW LATITUDES

TWILIGHT1. VARIATION OF TWILIGHT WITH LATITUDE:

HIGH LATITUDES

TWILIGHT

2. VARIATION OF TWILIGHT WITH DECLINATION:

Generally becomes larger when the Suns declination is higher

TWILIGHT2. VARIATION OF TWILIGHT WITH DECLINATION