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Time and the astronomical features which go with it form a vital element in our daily lives.

Even the solar day has to be calculated using a theoretical sun, because the real sun does not move at a constant relative rate.

a. the earth’s solar orbit is elliptical.

b. orbiting bodies behave in accordance with Kepler’s Law and so the speed of the earth’s motion around its orbit varies at different times of the year.

The most practical result of these factors is that the time of the sun’s meridian passage at Greenwich varies by up to 16 minutes ahead or behind midday.

The times of meridian passage are given daily in the NA.

If the earth rotates 360° in 24 hours, this can be broken down to a rate of 15° per hour or 4 minutes in time per degree of longitude.

In local time, Plymouth would be approximately 20 minutes behind London because the difference in longitude is about 5 degrees.

This is impractical for normal time keeping and time zones were introduced so that the same time would be kept in different places. The first company to use zone time was the GWR for their timetable.

The relative positions of heavenly bodies would be just the same if we were situated at another longitude, provided we amend the time at which we make our observation.

Although we use our longitude and the GHA of a body, the relative position is found by combining these to work out the difference, called the local hour angle (LHA).

LHA is equal to GHA minus longitude west or GHA plus longitude east.

Finally, a quick look at the relative motion of the moon illustrates the relative nature of astronomical observation. Although the moon completes its orbit of the earth in just over 27 days, when viewed from the earth, which is of course moving along its solar orbit, the event appears to take 29.5 days, the period of the lunar month.

The final diagram shows the phases of the moon, created as the moon changes position relative to the sun.

- 1. Grunt Productions 2007 TimeTime A brief by Lance GrindleyA brief by Lance Grindley
- 2. Grunt Productions 2007 Origins of TimeOrigins of Time • Apparent motion of the Sun around theApparent motion of the Sun around the Ecliptic takes one year.Ecliptic takes one year. • One rotation of the Earth on its axisOne rotation of the Earth on its axis (measured relative to the Sun) takes one(measured relative to the Sun) takes one solar day.solar day. • Period of the Moon’s revolution round thePeriod of the Moon’s revolution round the earth was one month (now arbitrary period).earth was one month (now arbitrary period). • A week is the length of time for the Moon toA week is the length of time for the Moon to progress from one phase to the next.progress from one phase to the next.
- 3. Grunt Productions 2007 Solar DaySolar Day • Interval elapsed between twoInterval elapsed between two successive transits of the Sun acrosssuccessive transits of the Sun across the same meridianthe same meridian • Apparent solar day is not a fixed length:Apparent solar day is not a fixed length: – orbits of planets are ellipsesorbits of planets are ellipses – the radius vector sweeps out equal areasthe radius vector sweeps out equal areas in equal periods of timein equal periods of time – time is measured around the plane of thetime is measured around the plane of the equator (inclined at 23equator (inclined at 2311 //22 oo to ecliptic)to ecliptic)
- 4. Grunt Productions 2007 Orbiting BodyOrbiting Body Body moves faster along orbit Equal areas swept by radius vector
- 5. Grunt Productions 2007 Local Mean TimeLocal Mean Time • System based solely on longitude.System based solely on longitude. • Based on earth’s rotation of 15Based on earth’s rotation of 15oo per hour.per hour. • Four minute time difference for everyFour minute time difference for every degree in longitude.degree in longitude. • Impractical for normal time keeping butImpractical for normal time keeping but used in astronomics to measure positionsused in astronomics to measure positions relative to the Observer’s Meridian.relative to the Observer’s Meridian.
- 6. Grunt Productions 2007 Action of SundialAction of Sundial GreenwichObserver’s Longitude Sun Dial Sun Ante meridian Sun’s Apparent Movement
- 7. Grunt Productions 2007 Use of Time in AstroUse of Time in Astro • NA lists the position of all heavenly bodiesNA lists the position of all heavenly bodies in terms of hour angle and declination.in terms of hour angle and declination. • Greenwich Hour Angle (GHA) is increasingGreenwich Hour Angle (GHA) is increasing at a rate of 0.25 min of arc per second.at a rate of 0.25 min of arc per second. • Hour angle relative to our position isHour angle relative to our position is referred to as the Local Hour Angle (LHA).referred to as the Local Hour Angle (LHA). • LHA is found from GHA plus longitudeLHA is found from GHA plus longitude East or minus longitude West.East or minus longitude West.
- 8. Grunt Productions 2007 The Local Hour AngleThe Local Hour Angle Local Hour Angle Pole Meridian of Star Equator Observer’s Meridian
- 9. Grunt Productions 2007 Hour AnglesHour Angles Earth Rotation Greenwich Meridian Apparent Rotation Westward Greenwich Hour Angle (GHA) Observer’s Meridian Longitude West Local Hour Angle (LHA)
- 10. Grunt Productions 2007 Longitude EastLongitude East Greenwich Meridian Greenwich Hour Angle (GHA) Observer’s Meridian Longitude East Local Hour Angle (LHA)
- 11. Grunt Productions 2007 Calculating LHACalculating LHA • Universal Time (GMT) must be used.Universal Time (GMT) must be used. • Obtain hourly prediction of GHA from theObtain hourly prediction of GHA from the Nautical Almanac.Nautical Almanac. • Obtain increment in hour angle based onObtain increment in hour angle based on minutes past the hour (buff pages of NA).minutes past the hour (buff pages of NA). • Apply correction for DR longitude subtracting forApply correction for DR longitude subtracting for W and adding for E.W and adding for E. • N.B. 360N.B. 360oo can be added or subtracted at anycan be added or subtracted at any time.time.
- 12. Grunt Productions 2007 Questions???Questions???

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