Chapter 0 Charting the Heavens The Fundatins f Astrnmy 0.1 The Obvius View Our Place in Space Humans live n a large rck called a planet Planets g arund giant balls f gas called stars Stars are tiny grains f sand within a galaxy Galaxies are tiny cluds within a nearly infinite universe Astrnmy is the study f the universe There are nly 3 things that were visible t ancient humans The Sun Nticed that the sun mved in the sky and linked t the temperature and weather patterns n earth t the sun s lcatin Mn Nticed that the mn repeats its appearance in an exact amunt f time and nticed that it affects things n earth as well Stars Nticed that sme are bright, sme are dim, sme mve a lt, sme dn t seem t mve at all, and that sme wuld be visible at times but nt visible at ther times Cnstellatins in the Sky The brightest stars bservable in ancient times were gruped tgether by humans Cnstellatins are cnfiguratins f stars that appear t mve tgether in the sky There are 88 ttal cnstellatins Patterns were discvered with these grups f stars Sme f them are visible year-rund Sme f them disappear and reappear at different times f the year Tracking these grups f stars helped them knw when t plant their fd at the start f spring and when t harvest their fd befre the winter came The cnstellatins were the first calendars invented by humans t help them survive the lng winters where they culd nt grw fd The Celestial Sphere The cnstellatins seemed t mve acrss the sky at night, but they mved tgether The Celestial Sphere is the imaginary sphere the ancients thught all the stars resided n that mved arund the fixed earth far ut in space Tday we knw that the Earth is spinning, causing it t appear like the stars are mving Rtatin is the spinning f an bject arund an internal axis Despite being inaccurate in its riginal frm, the Celestial Sphere is still used tday as a gd visual aid f hw t view the psitins f stars in the sky The Nrth Celestial Ple is the pint in the sky where the extensin f the Nrth Ple n Earth meets the Celestial Sphere The Suth Celestial Ple is the pint in the sky where the extensin f the Suth Ple n Earth meets the Celestial Sphere The Celestial Equatr is the circular plane thru the center f the Earth that is perpendicular t and halfway between tw Celestial Ples Celestial Crdinates Easiest way f lcating stars in the sky is t specify the cnstellatin and then rank the stars f that cnstellatin in rder f brightness using the Greek alphabet Alpha, Beta, Gamma, Delta, Epsiln, Zeta, Eta, Theta, Ita, Kappa, Lambda This methd wrks perfectly well fr naked-eye Astrnmy, but nt if we want t be much mre precise r if we have mre stars in a cnstellatin than the 24 Greek letters Celestial Crdinates are numbers that give the precise lcatin f anything the Celestial Sphere in the same way that GPS crdinates give us lcatin n Earth r an address gives the lcatin f a building within a city r twn
Measuring the Celestial Crdinates Declinatin is the extensin f the latitude lines fr earth and are used t measure the lcatin f an bject nrth r suth f the Celestial Equatr Measured in Degrees ( ), Minutes ('), and Secnds (") There are 60" in 1' f arc, 60' in 1 f arc, and 360 in a cmplete circle Celestial Equatr is 0, Celestial Ples are 90, Nrth is +, Suth is - Right Ascensin is the extensin f the lngitude lines fr earth and are used t measure the lcatin f an bject east r west f the Sun at the instant f the Vernal Equinx Measured in Hurs (h), Minutes (m), and Secnds (s) There are 60 s in 1 m, 60 m in 1 h, and 24 h in a cmplete circle Sun at instant f Vernal Equinx is 0 h, and increases t the east 0.2 Earth s Orbital Mtin Day-t-Day Changes The amunt f time it takes fr the Earth t rtate 360 arund its axis is 24 hurs. The Slar Day is the amunt f time it takes fr the Sun t g frm a specific lcatin in the sky t the exact same lcatin again This is the 24 hurs cmmnly referred t as a day Tday we knw that the Earth is als mving in a path arund the Sun Revlutin is the spinning f an bject arund an external axis Since the Earth revlves arund the Sun as well as rtates arund its axis, the earth has t rtate slightly further than 360 in rder t get the Sun back t the same lcatin as it was the day befre. This creates a different timeline fr the Sun as it des fr the stars, since the earth des nt revlve arund the stars as it rtates. The Sidereal Day is the amunt f time it takes fr a star t g frm a specific lcatin in the sky t the exact same lcatin again The Sidereal Day is 3.9 minutes shrter than the Slar Day This is why the stars appear shifted each day if charted at the same time (slar time). They g slightly further than 360 in 24 hurs Seasnal Changes Due t the revlutin f the Earth arund the Sun, the directin we see at night changes slightly each night. Over the curse f a year, the Sun appears t trace a circle thru the Celestial Sphere althugh it is actually the Earth that des the 360 turn arund the Sun The Ecliptic is the circle traced in the Celestial Sphere by the apparent mvement f the Sun thrughut the year. 12 Cnstellatins reside alng this circle. The Ecliptic Nrth Ple is 90 nrth f the Ecliptic The Ecliptic Suth Ple is 90 suth f the Ecliptic The Zdiac are the 12 Cnstellatins that the Sun sweeps thru ver the curse f a year The Zdiac signs are named fr the cnstellatin that we cannt see during that particular time f year due t the Sun being directly in ur line f sight t it In late February t early March, the cnstellatin Pisces cannt be seen due t the Earth facing it nly in the daytime The Zdiac sign f peple brn during this time is Pisces The Earth s axis des nt pint at 90 frm the plane it rbits the Sun, but is sits at 23.5 less This is why the Sun appears t mve nrth in the summer and mve suth in the winter. The Ecliptic is nt alng the Celestial Equatr, but rather at an angle f 23.5 abve it in the summer, and 23.5 belw it in the winter The sun appears t mve nrth in the summer and mve suth in the winter In reality, a different lcatin n the Earth pints directly at the Sun as ur tilted planet revlves arund it. The Summer Slstice is the nrthernmst pint the Sun reaches in the Celestial Sphere This ccurs n r near June 21 st (365.25 day revlutin) The Winter Slstice is the suthernmst pint the Sun reaches in the Celestial Sphere This ccurs n December 21 st (365.25 day revlutin) Page 2 f 5
The angle f sunlight striking each lcatin n earth varies with the mvement f the Sun alng the Ecliptic. The angle affects the cncentratin f heat/light striking the Earth When the Sun is in the nrthern hemisphere, the days are lnger and mre heat is delivered during the daylight, and when the sun is in the suthern hemisphere the days are shrter and less heat is delivered during the daylight This is the cause fr the 4 seasns we experience n mst f the Earth The distance frm the sun is nt the cause f the seasns, but the angle the Sunlight strikes the Earth. The Earth is actually clsest t the Sun in January and furthest in July There are tw instances where the Ecliptic crsses the Celestial Equatr Equinxes are when there is exactly the same amunt f day and night The Autumnal Equinx is when the Sun crsses frm nrthern t suthern hemisphere The Vernal Equinx is when the Sun crsses frm suthern t nrthern hemisphere Lng Term Changes The Earth revlves arund the Sun every 365.242 slar days The Trpical Year is the amunt f time it takes fr the Sun t g frm ne Vernal Equinx t the next. This is the 365.25 days cmmnly referred t as a year Tday we knw the Earth s nrth ple als revlves arund the Ecliptic Nrth Ple Precessin is the revlutin f the nrth ple Due t the gravitatinal trque n the Earth by the Sun and Mn, the earth takes lnger t revlve all the way back t the same psitin with respect t the stars than it des t revlve all the way back t the same psitin with respect t the sun This changes the directin the earth is facing each and every Vernal Equinx This means the Earth behaves just like a spinning tp that is wbbling It als means that the true nrth f the Earth pints at a different lcatin in space each year. Every 13,000 years the ple star changes between Vega and Plaris. The Sidereal Year is the amunt f time it takes fr the Earth t g frm a specific lcatin relative t the stars back t the exact same lcatin again The Sidereal Year is 20 minutes lnger than the Trpical Year, which makes it every 365.256 Slar Days At this rate, it takes 26,000 years t cmplete ne full precessin 0.3 The Mtin f the Mn Lunar Phases Unlike the reasn fr why the Sun mves in the sky, the reasn the Mn mves in the sky is because it revlves arund the Earth Phases are the different appearances f the mn as is revlves arund the Earth There are 2 wrds that describe whether the Mn is getting lighter r darker Waxing is when the Mn is getting lighter each night Waning is when the Mn is getting darker each night There are 5 different wrds t describe the phase f the Mn New is when the Mn is cmpletely dark Mn is directly between the Earth and Sun Crescent is when mre f the Mn is dark than light Mst f the lit part f the Mn faces away frm Earth Quarter is when the Mn is ½ Light and ½ Dark Mn frms a 90 angle with the Earth and Sun 1 st Quarter is used rather than Waxing Quarter 3 rd Quarter is used rather than Waning Quarter Gibbus is when mre f the mn is light than dark Mst f the lit part f the Mn faces tward the Earth Full is when the Mn is cmpletely light Mn is n ppsite side f the Earth as the Sun Page 3 f 5
Eclipses The Mn des nt prduce its wn light, but reflects the light it gets frm the Sun Its rbit des nt lie in the plane f the Ecliptic, but at a 5.2 angle with the Ecliptic which is what allws us t see the mn when it is full and see the sun when the mn is new despite ne being between the ther and the Sun The same face f the mn is always facing the Earth due t synchrnus rtatin which is when the rtatin and revlutin perids are identical The Sidereal Mnth is the amunt f time it takes fr the mn t g arund the Earth and back t its starting pint in the Celestial Sphere This happens every 27.3 days The Syndic Mnth is the amunt f time it takes fr the mn t g thru ne cmplete cycle f phases frm New Mn back t New Mn This happens every 29.5 days Because the Earth is als revlving arund the Sun at the same time, it takes lnger fr the Mn t get back t the same psitin with respect t bth the Earth and Sun Every s ften the Sun, Earth, and Mn will line up precisely and ne bject will cast a shadw nt the anther bject causing the 3 rd bject t disappear frm view This can nly happen during a new r full Mn A Lunar Eclipse is when the Earth s shadw sweeps acrss a full Mn blcking the Sun s light frm reaching the Mn During a Lunar Eclipse, the entire prtin f earth that can see the Mn bserves the same eclipse A Partial Eclipse is when nly part f the bject is eclipsed A Ttal Eclipse is when the entire bject is eclipsed A Slar Eclipse is when the new Mn s shadw sweeps acrss the Earth blcking the Sun s light frm reaching the Earth During a Slar Eclipse, there are different views seen frm Earth due t the prperties f light and the distance frm the Mn t the Earth The Umbra is the darkest part f the shadw f the Mn where all f the light frm the Sun is blcked The Penumbra is a lighter part f the shadw f the Mn where nly sme f the light frm the Sun is blcked If the mn is nt clse enugh t the Earth during the eclipse, n part f the Sun will be cmpletely eclipsed as there is n part f the Umbra that reaches the surface f the earth An Annular Eclipse is when there is n Umbra n the Earth Annular cmes frm annulus which means ring, which is what an Annular Eclipse lks like A ring f the sun can be seen alng the uter edge f the mn in the sky 0.4 The Measurement f Distance Directins are imprtant in Astrnmy, but in rder t make predictins r determine ur exact place in the Universe, the distances are als needed t d calculatins Fr centuries, humans have used their knwledge f Euclidian Gemetry t calculate distances that were unable t be measured directly. This methd is still used tday in many prfessins. Triangulatin is the methd f cnstructing a Right-Triangle using 2 measurements alng a knwn distance at the base f the triangle t calculate the distance t the bject The Baseline is the name f the side in the triangle that is the knwn distance between the tw lcatins being used t calculate the angle between them Using basic Trignmetry, all ther parts f the triangle can be determined very accurately with just the knwledge f the angle between the tw pints alng the baseline and the baseline itself The distance t the bject and the angle alng the baseline have an inverse relatinship As the distance t the bject increases, the angle will decrease Page 4 f 5
Our eyes and brain d this exact same calculatin between the view f ur left and right eyes If yu find an bject far away in yur line f sight and place yur finger at arms-length in frnt f yu and clse ne eye, yur finger will appear t be in a certain psitin. If yu then clse that eye and pen the ther, the finger will appear t be in a different psitin relative t the bject behind it Yur brain takes these tw images and cnstructs an accurate image f the exact lcatin f the finger in bth the angle and distance. Parallax is the apparent displacement f a fregrund bject relative t the backgrund as the bservers lcatin changes. In 200 B.C. the Greek philspher Eratsthenes used triangulatin t calculate the diameter f the Earth frm the length f shadws that tw twers cast n the grund n the Summer Slstice I. He measured the length f the shadws that tw identical twers in different lcatins n the Earth n the lngest day f the year. Using triangulatin, he was able t calculate the angle f the Earth between the tw Knwing that the Earth was basically a circle (Aristtle prved in 300 B.C.), he was able t use the gemetric idea f similar shapes t calculate the diameter f the Earth He calculated the circumference and radius f the Earth t be 40,000 km and 6,366 km Tday with ur mdern technlgy we use 40,070 km and 6,378 km! 0.5 Science and the Scientific Methd Science is a prcess fr investigating the physical wrld based n natural laws and bserved phenmena. Prgress in science is ften slw and intermittent and can require a great deal f patience befre significant prgress is made. The earliest descriptins f the universe were based n mythlgy and imaginatin and made little attempt t explain the wrkings f the heavens in terms f testable ideas that cme frm cllected infrmatin Eventually certain peple discvered that careful bservatin and testing f ideas was much mre valuable in explaining and predicting bservable events A Thery is a framewrk f ideas and assumptins that used t explain sets f bservatins and make predictins abut the real wrld. Theries must be cntinuusly tested and changed if there is new bservable evidence that cannt be explained using the current thery A Theretical Mdel is visual r mathematical explanatin that accunts fr the all f the knwn prperties and bservatins f a particular tpic The mdel will make additinal predictins r even predict hw it wuld change under different circumstances, which can then be tested and verified r rejected The Scientific Methd is a sequence f investigatin, cllecting bservable infrmatin, therizing, and experimenting t learn relatinships in the natural wrld. It is a cntinuus prcess that can be mdified and changed at any pint shuld new evidence r smething that cannt be explained by current theries is discvered. Scientific theries have several defining characteristics They must be testable, and can be fund invalid if new evidence cntradicts them They must be cntinually tested and their cnsequences tested as well They shuld be simple and the simpler the idea the better Occam s Razr is a principle that states if there are tw different theries fr the same phenmena that the best explanatin is the mst simple ne Page 5 f 5