2 Terms to KnowRotation: Earth completes one rotation every 24 hours giving us day and night.
3 Terms, cont.Revolution: Earth completes one revolution around the sun every 365 ¼ days.
4 Terms, cont. Revolution is not a perfect circle, more of an elliptic. When revolution brings earth closest to sun: perihelionWhen revolution takes earth furthest from sun: aphelionPoint 1: Earth at aphelionPoint 2: Earth at perihelionPoint 3: Sun
6 Terms, cont.Ecliptic plane: This is the level, or plane, of Earth’s orbit around the sun…Earth’s orbital path.Astronomically, this is the primary plane referred to when discussing the revolution of other planets around the sun
7 Terms, cont.Inclination: Relative to the ecliptic plane, Earth is tilted 23.5° from vertical at all times.
8 Terms, cont.Polarity: Because Earth is always tilted 23.5°, it’s axis is always pointed in the same direction…towards the star Polaris.Because of this regular tilt, when the Earth revolves around the sun, the North pole is either tilted towards the sun (June) or tilted away from the sun (December)DO NOT confuse the term “polarity” with magnetism
9 Earth’s EquinoxesEquinox: Earth is positioned so that its is neither towards nor away from the sun…giving the entire planet equal amounts of day and night.Occurs twice each year, around March 20 and September 22.
10 Earth’s SolsticesSolstice: Earth is positioned where its tilt is either towards or away from the sun.Occurs twice a year, just like the Equinoxes, around June 21 and December 21.The direct rays from the sun hit the earth at 23.5°N on June 21, while the tangent rays hit the earth around 66.5°N and 66.5°S.
14 Video on Seasons http://www.youtube.com/watch?v=taHTA 7S_JGk Powers of ten:
15 Solar Angle Hess, McKnight’s Physical Geography, 10 ed. 17-20 pp. Lesson 11Solar AngleHess, McKnight’s Physical Geography, 10 ed.17-20 pp.
16 Sun DeclinationRecall, the vertical rays (direct rays) are those from the sun that strike the Earth at a 90° angle.The location of these vertical rays changes throughout the yearLocated at the Tropic of Cancer during the summer solsticeLocated at the equator during the equinoxes.The latitude at any given time of the year where the sun’s vertical rays strike the Earth is known as the sun declination.
17 Sun Declination, cont.The sun’s declination can be plotted on a graph, known as an analemma (next slide)The sun’s declination is plotted along the vertical axisThe days of the year are along the analemma itself
18 Mars’ analemma, top right…Earth’s analemma bottom right
19 Solar AltitudeSolar altitude is the elevation of the sun in the sky at noon local timei.e. the angle of the noon sun above the horizonCan be calculated mathematically:𝑆𝐴=90° −𝐴𝐷where SA is the solar altitude and AD is the arc distanceArc distance (AD) is actually the difference between the latitude your at and the declination of the sun at that time of year.Let’s look at an example…
20 Solar Altitude Example Suppose it’s noon and we’re outside on campus during a snowstorm on January 12th. The clouds and snow showers are blocking the sun from being visible. Even though we can’t see the sun, what is the solar altitude (sun’s elevation above the horizon)?
21 Solar Altitude Example, cont. The latitude of Muncie is 40° 11’ 36” N.Looking at our analemma, we find that on January 12th, the sun’s declination is 21.5° S.The arc distance (AD) can be found by subtracting the sun’s declination from our latitude:40° ─ −21.5° = 61.5° = ADThe reason there is a negative in front of the 21.5° is because the sun’s declination was “S” and the sun is over the southern hemisphere.
22 Solar Altitude Example, cont. Now that we know AD, plug into the equation we were given:𝑆𝐴=90°−𝐴𝐷𝑆𝐴=90°−61.5°𝑆𝐴=28.5°Therefore, if we could see the sun through the clouds and snow, it would only be 28.5° above the horizon.For additional examples, see page 51 and 53 in your lab manual.
23 Tangent Rays and Daylight/Darkness If we know the latitude of declination for the sun (where it’s direct rays strike the Earth; from the analemma), then we can easily find the latitude of the tangent rays (those rays that skim past the earth).Simply use this equation:𝑇𝑅=90°−𝐷𝑒𝑐Where TR is the latitude of the tangent rays and Dec is the sun’s declination
24 Tangent Rays and Daylight/Darkness, cont. From our previous example, we found that the declination of the sun on January 12th is 21.5° S. So, plugging that in:𝑇𝑅=90°−𝐷𝑒𝑐𝑇𝑅=90°−21.5°𝑇𝑅=68.5°So, we know that those latitudes above 68.5° N and 68.5° S receive either 24 hours of sunlight or 24 hours of darkness…but which is which?
25 Tangent Rays and Daylight/Darkness, cont. For the Northern Hemisphere, remember this:If the day is between March 20 and September 22, then those areas north of the latitude of the tangent rays are experiencing 24 hours of daylight.If the day is between September 23 and March 21, then those areas north of the latitude of the tangent rays are experiencing 24 hours of darkness.