Cyclical movements of glowing objects in skies were the mainhistorical stimulus for the study of astronomy. The movements becamethe topic of intensive study by many ancient cultures, also if theycould not analyze the activities properly. Ultimately, evaluation of thesemotions resulted in some of the many important advancements inthe background of modern science. In this guide, we describe some of thesemotions as they can have been viewed by old astronomers butexplain them from a modern-day perspective.

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A. Motions of shining Objects

The table listed below lists celestial activities which are quickly detectable bysomeone on the earth without telescopes. OBJECTPERIODMOTIONALLDaily ("diurnal")Rotation WestwardSUNAnnual (365 days)(a) 1 degree/day Eastward*(b) North/South* MOON**Monthly (29 days)(a) Eastward, N/S* (b) Phase change PLANETS (5)**Months-YearsGenerally Eastward, however withWestward loops*

*Except for the diurnal motion, movements are measured with respect come the stars.The diurnal movement is measured through respect come the regional horizon. **The Moon and the planets relocate on paths close to to, however not identicalwith, the annual path that the sunlight seen versus the stars(the "ecliptic")Experiencing the motions civilization are familiar with the movement of the Sun during the day. Thatis the most noticeable manifestation that the "diurnal motion" of the wholesky. Yet the other motions detailed are less conspicuous and also so slowthat if girlfriend aren"t a practicing amateur astronomer, girlfriend probablyaren"t mindful of them. If you"re patient for around 20 minutes, friend can quickly detect thediurnal wheeling rotation that the totality night sky by comparingthe position of the stars come a fixed foreground thing (e.g. A tree).If you have a digital camera capable of taking exposures much longer than afew minutes, you have the right to readily take pictures showing star trails,like the one in ~ the peak of the page.The best way to visualize celestial motions is in a planetarium orwith a good computer sky simulation program. We will usethe Starry Nightsimulator in class.But a faster way to endure the diurnal movement is to see one ofthe numerous excellent time-lapsevideos now easily accessible on the Web.Celestial motions might be complicated, however they are repeatableover periods of months to years, and their cyclic nature to be quicklyrecognized by numerous cultures. It was this feature that encouragedpeople to search for a deeper knowledge of them, specifically inview the what they can portend for themselves. The cycles additionally naturally came to be the communication of handy calendarsystems. For instance, western calendars are based upon 7-day weeks(one day because that each that the bright moving lights in the sky: the Sun,Moon, and also five planets), 30-day months (one lunar cycle),and 365-day years (one bicycle of the sun in motion againstthe stars).

B. Explanation that Motions

In the rest of this guide, we define these phenomena indigenous amodern clinical perspective. That took many centuries forastronomers to arrive at the correct interpretation explained here.Ancient Greek astronomers construed much the this after number of hundredyears that work, however the expertise was lost and also only rediscovered duringthe Renaissance, 1300 years later.The key to complete understanding that celestial movements was introducedby the Greeks: mathematics. The Greeks built mathematical models the the sky based onplane and spherical geometry which castle haddeveloped. This reduce a bewildering array of complexphenomena come a lot simpler collection of mathematical concepts. Although it is fashionable this particular day to slam "reductionism" inscience, there would, in fact, have actually been very small progress inunderstanding nature (or in convert that expertise to usefultechnologies) without the significant simplifying insights ofreductionism. The noted motions the celestial objects are developed bytwo totally different effects: Intrinsic activities of the objects themselves v respectto one an additional The motion of the observer, or the communication on whichhe/she is standing---in this case, the Earth since the activities of objects in the sky have the right to be induced through movementof the observer, we contact them apparent motions. The motions discussed in this guide are every "apparent motions" inthe 2nd category and also are induced through the fact that: THE earth WE LIVE top top IS A ROUND, TILTED,SPINNING, relocating PLATFORM. Although that is basic to state, it is very daunting for mostpeople to visualize this situation. Girlfriend instinctively feelyourself to be "upright" and at remainder on a flat, stationaryEarth, but your instinct below is seriously misleading. This isone that the key reasons that took humans nearly 2000 years tofully expropriate this explanation, which ended up being the very first major discoveryof the scientific period (by Copernicus ca. 1540, watch Study guide 6). The movements of the Moon and also the planets are explained onthe Lunar Motions page andin Study guide 5, respectively.

C. Impacts of Earth"s Shape and Spin

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Earth is a sphere, with a diameter that 7900 miles. Half of planet is always in sunlight; half is alwaysin shadow. We suffer night ~ above the shadow side (away native Sun). Daylight = sunlight scattered through the Earth"s atmospherewhenever the sun is above the horizon. The daylight glare overwhelmsthe irradiate of the planets and also stars, so we cannot check out them (though theyare, that course, always there). The glare rapidly declines as the Sundips listed below the horizon ("dusk"). "Civil twilight," the allude at whichsunlight is simply barely perceptible, occurs as soon as the sun is 6 degreesbelow the horizon.Earth spins ~ above its axis through respect to the stars when in23 hours, 56 minutes (one "sidereal day").This reasons theapparent diurnal rotation that the entirety sky. (The universe is notmoving approximately the earth once every day.)If the earth were assumed not to it is in spinning, climate thediurnal motion of the skies would necessarily imply that it to be atthe center the the universe. This was the first assumptiondiscarded by Copernicus on his way torejection of the ancient "geocentric" cosmologies. Earth"s turn is counterclockwise (eastward) as checked out from aboveEarth"s N pole; this way that the apparent rotation of the sky seenfrom Earth"s surface is westward. The left-hand dashboard of diagram over shows planet viewed from over itsNorth Pole. The planet rotates counterclockwise in this diagram,carrying observers v it. The positions wherein observers top top theequator room experiencing sunrise, noon, sunset, and also midnight aremarked. Mean local times that day matching to these points are6 AM, 12 noon, 6 PM, and also 12 midnight, respectively.The right hand panel reflects the regional "horizon plane,"which is the plane "tangent" to earth at your location. You deserve to see(in principle) objects above the aircraft but not below it. Keep in mind thathalf the the whole "celestial sphere" (seeStudy overview 3) is always over yourhorizon plane. However, because the planet is spherical, the horizonplane (and the clearly shows hemisphere) is different in ~ eachlocation ~ above the Earth"s surface. The horizon aircraft sweeps across the sky as planet spins.Astronomical objects show up to relocate in the opposite direction(shown through the arrows in the figure). Objects increase over the easternhorizon and set toward the west horizon. By combining the principle of the horizon plane with the definitionof regional time in the left-hand panel, you deserve to visualize whatparts of the skies are observable at any type of given time at a givenlocation ~ above the Earth.

D. Results of Earth"s motion in Orbit

Earth is a planet relocating in orbit roughly the sun Its orbit is nearly circular (the distance to the sunlight variesonly 3.4%), v a typical radius of 150,000,000 kilometres or 93,000,000miles. The average radius is defined to it is in the AstronomicalUnit (AU). That orbit lies in a plane dubbed the "ecliptic" plane. Seenface-on, the orbit is technically an ellipse but deviates onlyslightly indigenous a circle. Watched edge-on, the orbit is a slim line. planet orbits the sun in 365.25 days (one year) moving at~66,000 mph. Its motion is counterclockwise as viewed from abovethe north pole.
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Stars clearly shows at night are those "opposite" the Sun. See the twofigures above. The night side of planet is that opposite the Sun. So,in May, the constellation Scorpio will certainly be significant in the night sky,while in November, it lies in the direction that the Sun and thereforeis not visible due to the fact that of the daytime atmospheric glare. Warning! the illustrations in this guide,and many others friend will watch in this course one of two people in the message or inclass, are grossly distorted and also not to scale! Don"t take it themliterally. In contrast to the drawing over of the Earth"s orbit, thereal orbit is 100 times the diameter of the Sun; the earth itself is100 times smaller sized than the Sun; the stars room vastly far-off from theEarth"s orbit; and also the stars in a provided constellation space notnecessarily near one another in space. Obviously, no one might produce or sensibly watch a figurelike this attracted to yes, really scale. We are viewing Earth"s orbit here from one inclination suchthat it looks extremely elliptical, whereas that is nearly circular seenface-on. The Earth"s motion about the sunlight is counterclockwise inthe drawings above (drawn native a viewpoint that is north of the Earth"sorbital plane), so that in August, the Earth"s position would benearest the illustration of Capricornus in the diagram. Earth"s orbital movement produces an obvious eastward "reflexmotion" of the sun in the sky versus the stellar reference framework asseen native the Earth. Native the drawings, we discover that the sunlight in November would certainly be seenin projection in the direction that Libra, when in December, 30 dayslater, the earth has moved such the the sun is viewed in projectionagainst Scorpio, i beg your pardon is roughly 30 degrees east the Libra.Therefore, the median "reflex motion" of the sun with respect to thestars as watched from earth is 1 degree per work eastward. The ecliptic path: The Sun"s yearly path checked out from the moving Earth versus thestars is dubbed the "ecliptic path." This is just the projection ofthe ecliptic aircraft on the celestial sphere. The "Zodiac" is the collection of constellations v whichthe ecliptic course passes. Those room the only constellations whichhave been portrayed in the drawings above. Of course, there space 76other constellations not shown. The only expensive significance of the 12 Zodiacalconstellations, therefore, is accidental: they happen to lied on theprojection that the plane of the Earth"s orbit. They space not thelargest, brightest, or most influential constellations (in reality a numberare quite inconspicuous). Practice exam question: which Zodiacal constellation will certainly be finest seen in ~ midnight in August? "Solar" vs. "sidereal" days: The day-to-day motion of the planet in itsorbit method that the earth must turn a little more 보다 onceon the axis to lug the Sun back to the point of regional "noon" (i.e.halfway in between rise and also set). The extra quantity is 4 minutes, onaverage. The result of Earth"s motion during one sidereal work isillustrated here. This accounts because that the difference between the sidereal spinperiod of the planet (23 hours, 56 min) and the averageelapsed time in between successive noons (24 hours, the"solar day"). Our plain clocks, that course, are set such the thetime between successive noons is defined to it is in 24 hours. Butthe stars come back to the same place in the sky as seen fromEarth 4 minutes previously each day. This means, because that instance,that the stars you view at 11 pm in a given month have the samelocation with respect come the horizon as they did at 9 pm one monthearlier or 7 PM 2 months earlier.

E. Tilt of Earth"s Axis

The polar rotation axis the the planet is no perpendicular toits orbit plane. it is tilted by 23.5 degrees native the vertical,or, equivalently, the is tilted 66.5 degrees out the the plane. Watch figurebelow (again, exaggerated for clarity):
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The rotation axis is addressed in direction with respect to thestars, not through respect to the Sun. As Earthorbits the Sun, the axis proceeds to suggest in the same directionin 3D space. This indicates that the north pole is sometimes"tilted toward" the Sun, sometimes "tilted away." In the illustrationabove, the north Pole is presented at its maximum tilt far from the Sun(assuming the phibìc Pole is in ~ the top). The addressed axis direction that the Earth during a year is illustratedin the drawing in section F below. This, in turn, indicates that the Sun, as viewed from the Earth,will appear to move north and also south the the celestial equatorthrough the year through a maximum of + or - 23.5 degrees. In the drawingabove the earth is located such the the sun will be checked out at that mostsoutherly position during the year.The full amplitude the the Sun"s N/S swing across the celestialequator is 2 x 23.5 = 47 degrees. Equivalently, the ecliptic path is skinny 23.5degrees to the celestial equator. The complying with diagram showsthe ecliptic and the equator on the celestial sphere (see overview 3 for the meaning of the celestialsphere).
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The ecliptic crosses the celestial equator at two pointscalled equinoxes. when the sunlight is at an equinox, night and day space each 12 hours long at all latitudes. The "Vernal Equinox" in the north hemisphere occurs approximately March 21, when the "Autumnal Equinox" occurs approximately September 21. The sun is in ~ its biggest distance from the equator (23.5 degrees) at the solstices ("sun stationary"), i beg your pardon are around June 21 and Dec 21. At these times, one hemisphere experiences its longest day, the other its shortest. The diagram below shows the place of the sun on the celestial round with respect come the equator plotted versus date. The longest and shortest days in the north hemisphere occur at the June and also December solstices, respectively. (The labeling in the chart is for the northern hemisphere; the periods are reversed in the southern hemisphere.)
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F. Astronomical Effects ~ above the Weather

The Sun"s north/south distance from the celestial equatordetermines: The number of hours of daylight in ~ a offered latitude and The angle in ~ which sunlightstrikes Earth"s surface ar at a provided latitude. More energy isincident per unit area per 2nd on the surface the closer space the raysto perpendicular to the surface. The Sun"s angular street from the celestial equator thusdeterminesinsolation, or the quantity of solar energy deposited top top a provided areaof the Earth"s surface throughout 24 hours at a given latitude. This differential heating is responsible because that the seasons.The "official" beginning dates the spring, summer, autumn, and wintercorrespond to the Vernal Equinox, Summer Solstice, fall Equinox,and Winter Solstice, respectively. The diagram listed below shows how the hours of daylight (represented bythe length of the white lines on the illuminated half of the Earth)and the solar angle of incidence at various latitudes correlate withthe date. A close-up the the case is presented here.
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The adjust in the geographical shadow distribution caused by thetilt is quite dramatic (even though the shadow constantly covers exactlyone half of the Earth"s surface). Listed below are two pictures of the means theshadow is dispersed at two different times that year. The Earth"ssurface moves eastward (toward the right in the diagrams) with theshadow together the planet rotates. Girlfriend can instantly tell from the imagewhich latitudes room receiving much more sunlight in a 24 hour period.Click ~ above the thumbnails for an broadened view.

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Hereis a quite timelapse video compilation of photos from space of theEarth"s surface at the autumnal Equinox in 2013. Therefore, the periods are led to by the tilt that the Earth"spole come its orbit plane, no its distance from the Sun. If the seasons were a distance effect, winter, for instance, wouldoccur simultaneously in both the southern and also northernhemispheres. Yet instead, the periods differ through 6 month in the twohemispheres. There is a small annual adjust in the distance ofthe earth from the Sun led to by the finite ellipticity of Earth"sorbit. This amounts to just + or - 0.017 AU. The change in distancehas a real, yet small, effect on the seasons. The planet is nearestthe sun in January, one of the coldest month in the northernhemisphere. The seasonal changes because of this are milder in the northernhemisphere and also larger in the southerly hemisphere 보다 they would certainly havebeen if Earth"s orbit were perfectly circular. regional Weather Extremes:The change in insolation in ~ the mid-latitudes (likeCharlottesville"s) would certainly not through itself cause the extremes that weatherwe experience right here (e.g. Frigid winter temperatures). Instead, it ismixing through wind currents of air masses from differentlatitudes the produces the extremes. In the case of winter, thenorth polar regions experience perpetual night beginning in Septemberand lasting until March. The resultant drastic atmospheric coolingcreates the icy air masses that have the right to produce an extremely cold weather in the USif they move over us. The Milankovitch EffectThe polar tilt and the properties of the Earth"s orbit changeslightly through time over long periods (10"s of countless years)because of the gravitational results of the other bodies in the solarsystem. Little changes in the polar tilt, the size and shape ofEarth"s orbit, or the time of closest technique to the Sun have the right to havecumulative effects on heater of the atmosphere.One the the important impacts isprecession, questioned on the Lunar motions page, i beg your pardon changesthe timing of the solstices through respect come the Earth"s distancefrom the Sun. There is good geophysical evidence for a correlationbetween such astronomical changes and also major changes in climate over lengthy time periods, particularlyin the type of iceages. This is known as the "Milankovitch Effect."Astronomical and atmospheric effects on climate are questioned furtherin Study overview 19.

G. Impacts of Intrinsic Lunar Motions

See Lunar activities andTheir Consequences

Reading because that this lecture:Bennett textbook: Ch. 2.1, 2.2 Study guide 4Lunar Motions and also Their ConsequencesPuzzlah ready exercise. (Work alone or in a tiny group.) measure up the angular diameter that the Sun as follows. Youcan do this on any kind of day (clear or partially cloudy) as soon as you can see thedisk the the Sun. Don"t look directly at the sun. Instead put yourhand (palm the end & fingers together) in prior of her eyes in ~ arm"slength. Close one eye. Then, carefully fold under fingers, keeping theSun"s light covered until girlfriend can"t eliminate any much more fingers withoutletting sunshine pass. Remember that your table of contents finger will certainly subtendabout 1 degree in broad when organized at arm"s length. How vast is the sun in degrees? Repeat this practice for the star cluster the Pleiades (in the constellation Taurus) regarded on a clean night. I m sorry is larger, the sun or the Pleiades? If the totality sky (both hemispheres) consist of 41,000 square degrees, how plenty of times over might you to the right the Sun"s noticeable disk right into the entirety sky?Reading for next lecture: Bennett textbook: CH 2.3 (lunar phases, eclipses), 3.1 (ancient astronomy)Study overview 5Aztec Calendar StoneLunar Motions and Their ConsequencesOptional reference on Mayan astronomy: Skywatchers ofAncient Mexico by Anthony F. Aveni (Univ. That Texas Press,1980/97).Puzzlah preparation Questions

Web links:Spectacular star trailphotos through L. HarrisonThanks come the capabilities of contemporary digital cameras because that takinghundreds of pictures in sequence, there are now scores the beautifultime-lapse videos the the night sky, the Earth, and also views indigenous International an are Station made by both professional and also amateurastronomers. Web links to some great video sites are offered onthe ASTR 1210 internet Links Page.InteractiveEarth & Moon viewer (includes software & add"l links)JPL Tutorialon planet motions and astronomical referral systemsIce periods (Wikipedia)Lecture on the Milankovitch effect (J. Frogel, OSU)MilankovitchCycles (Wikipedia)

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