It’s All In The Stars: Earth’s Seasons and Human Navigation

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Celestial Navigation Celestial navigation, also known as astronavigation, is a position fixing technique that has steadily evolved over several thousand years to help sailors cross featureless oceans without having to rely on estimated calculations to help them know their position on the ocean.

How is it done? Celestial navigation uses "sights," or angular measurements taken between a visible celestial body (the sun, the moon, a planet or a star) and the visible horizon. At any given instant of time, any celestial body is located directly over only one specific geographic point, or position on the Earth, whose address is described by latitude and longitude.

Measurement Accurate angle measurement evolved over the years. One simple method is to hold the hand above the horizon with your arm stretched out. The width of a finger is an angle just over 1.5 degrees

Accuracy The need for more accurate measurements led to the development of a number of increasingly accurate instruments, including the kamal, astrolabe, octant and sextant. kamal astrolabeoctantsextantkamal astrolabeoctantsextant

Sextant and Octant The sextant and octant are most accurate because they measure angles from the horizon, eliminating errors caused by the placement of an instrument's pointers, and because their dual mirror system cancels relative motions of the instrument, showing a steady view of the object and horizon.

Using the Sexton to Measure the Altitude of the Sun Above the Horizon

Today Modern practical navigators usually use celestial navigation in combination with satellite navigation to correct a dead reckoning track, that is, a course estimated from a vessel's position, angle and speed. satellite navigationdead reckoning satellite navigationdead reckoning

Latitude LatitudeLatitude was measured in the past either at noon (the "noon sight") or from Polaris, the north star (assuming it is sufficiently visible above the horizon, which it may not be in the Southern Hemisphere). Polaris LatitudePolaris

Longitude LongitudeLongitude can be measured in the same way. If one can accurately measure the angle to Polaris, a similar measurement to a star near the eastern or western horizons will provide the longitude. Longitude

Seasons Spring, Summer, Fall, Winter

Definition of a Season A season is a division of the year, marked by changes in weather, ecology, and hours of daylight. yearweatherecologydaylightyearweatherecologydaylight Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution. EarthSuntilt of the Earth's axisEarthSuntilt of the Earth's axis

North vrs South In temperate and subpolar regions generally four calendar based seasons are recognized: spring (adj. vernal), summer (adj. estival), autumn (adj. autumnal), and winter (adj. hibernal). temperatesubpolarcalendarspringsummerautumn wintertemperatesubpolarcalendarspringsummerautumn winter

Tropics In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season, because the amount of precipitation may vary more dramatically than the average temperature. tropicalsubtropicalrainymonsoondry season precipitationtropicalsubtropicalrainymonsoondry season precipitation

Earth The seasons result from the Earth's axis being tilted to its orbital plane; it deviates by an angle of approximately 23.5 degrees. Earthaxistiltedorbital planedeviates degreesEarthaxistiltedorbital planedeviates degrees The effect of axis tilt is observable from the change in day length, and altitude of the Sun at noon (the culmination of the Sun), during a year day lengthaltitudenoonculminationyearday lengthaltitudenoonculminationyear

Aphelion and Perihelion Seasonal weather differences between hemispheres are further caused by the elliptical orbit of Earth. elliptical orbit elliptical orbit Earth reaches perihelion (the point in its orbit closest to the Sun) in January, and it reaches aphelion (farthest point from the Sun) in July. perihelionSunaphelionperihelionSunaphelion

Visualize This:

Weather Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year.

Winter to Summer

Solstice and Equinoxes In astronomical reckoning, the solstices and equinoxes ought to be the middle of the respective seasons, but, because of thermal lag, regions with a continental climate often consider these four dates to be the start of the seasons as in the diagram, with the cross-quarter days considered seasonal midpoints. astronomicalsolsticesequinoxescross-quarter daysastronomicalsolsticesequinoxescross-quarter days

Seasons! hill.com/sites/ x/student_view0/c hapter2/seasons_interactive.html hill.com/sites/ x/student_view0/c hapter2/seasons_interactive.html