DOCSLIB.ORG

The Celestial Sphere, the Coordinates System, Seasons, Phases of the Moon and Eclipses

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Celestial Sphere, the Coordinates System, Seasons, Phases of the Moon and Eclipses The celestial sphere, the coordinates system, seasons, phases of the moon and eclipses Chapters 2 and S1 The celestial sphere and the coordinates system Chapter S1 How to find our way in the sky? Let’s start with the Earth Coordinate System Latitude: N-S of the equator, Longitude: E-W along equator From Earth to Space The Celestial Sphere • The Celestial Sphere: An imaginary sphere of infinite radius centered on Earth. • The extensions of the Earth North Celestial Pole North and South Pole define the North and South celestial poles. • The projection of Earth equator defines the Celestial equator. • Celestial Sphere can then be divided into a grid, just like North celestial poleNorth the Earth is divided into a grid of latitude and longitude. Equator Celestial South Celestial Pole The Celestial Sphere: Motions • Stars, planets and Sun are “attached” to this imaginary sphere. • As the Earth rotates, the celestial sphere (with the stars attached to it) appears to rotate in the opposite direction. • To explain the daily motions of the sky you can imagine the sphere rotating once in 23 hours 56 minutes (using a star as reference). Celestial Sphere: Measuring Angles Longitude (E – W along Equator) Right Ascension (RA) Latitude (N – S of Equator) Declination (Dec) The celestial coordinate system North Celestial Pole RA, measured in hr, min, sec (0 to 24 hours) 1 hour = 60 min 1 min = 60 sec (1 hour = 15 degrees of Earth rotation) orth celestial poleNorth Dec, measured in degrees, arcmin, arcsec (0 celestial equator, +90 north hemisphere, -90 south hemisphere) Equator Celestial 1 degree = 60 arcminutes 1 arcminute = 60 arcseconds South Celestial Pole The use of RA and Dec to locate objects in the celestial sphere There are two coordinates that allow to locate an object in the sky: Azimuth and Altitude. Their value depends in the location of the observer Azimuth: Use as reference the north direction (close to Polaris) and the range of values is from 0 to 360 degrees. 0 degrees is N, 90 degrees E, 180 is S and 270 is W. Altitude: Use as reference the horizon. The range of values is from 0 degrees (horizon) to 90 degrees (zenith) Locating the star Vega and the Sun in the celestial sphere Ecliptic: Apparent annual path of the Sun in the celestial sphere The Sun crosses the celestial equator on March 21 (Spring equinox) and on September 21 (Fall equinox) The Sun reaches a declination of +23.5 degrees on June 21 (Summer solstice) The Sun reaches a declination of – 23.5 degrees on December 21 (Winter solstice) Locating Polaris (North star) in the celestial sphere Locating Polaris: RA: 0h 31m 49.084s Dec: +89d 15’ 50.79” Using two stars in the Big Dipper (Ursa Major) constellation called the Pointers” Angular Size Angular size of an object depends on two parameters The physical size of the object The distance to the object Angular size is measured in units of angle (degrees, arcmin and arcsec) Physical Size Angular size = Distance More specifically (See page 30, Mathematical Insight 2.1) Angular Size = Physical size 360 degrees 2 x Pi x distance Angular size = Physical size x 360 degrees/ (2 x Pi x distance) Example: Physical size of the Moon Angular size = 0.5 degrees Distance = 380,000 km Angular Units Estimating angular sizes Practical Measurements • The Moon and the Sun, coincidentally, have nearly the same angular size, about 0.5 degrees. • The Moon is about 380,000 km away but only 3,300 km diameter • The Sun is 150,000,000 km away and about 1,400,000 km diameter Celestial Sphere and the Observer Horizon: flat plane where observer stands Zenith: the point directly above an observer Nadir: the point opposite to the zenith An observer can see only half of the celestial sphere from any location on Earth Apparent Motion of Stars Earth rotates from W-E celestial sphere seems to rotate E-W. Depending on our location, we’ll see some stars rising on the east and setting on the west. Depending on our location, some stars never set. Those stars are called circumpolar stars. For someone standing at the equator, all stars rise and set. For someone standing at the poles, all stars are circumpolar. Observer located at the equator Orientation of the sky relative to Rotating the diagram make it the celestial sphere, for an easier to visualize the local sky observer at the Earth’s equator at the equator Meridian: The circle that passes through the zenith and the two celestial poles Observer located at the north pole Observer located at latitude 40 degrees N Rotating the diagram so the zenith is The latitude is the angle from the zenith to up make it easier to visualize the the Earth’s equator. “Up” point to the circle local sky. on the celestial sphere with declination +40 The blue scale along the meridian degrees shows altitudes and directions in the Notice that the south pole is below the local sky. horizon and invisible for an observer located Notice that the altitude of the north at 40 degrees N latitude celestial pole is 40 degrees which correspond to the latitude of the place How can we estimate our latitude? Remember that the angle between the horizon and the object is called altitude The altitude of the north celestial pole, give us our latitude. Polaris is close to the north celestial pole. By estimating the altitude of Polaris we can estimate the latitude of the observer. The path of the sun on the equinoxes and solstices at latitude 40 degrees north (Latitude of Gainesville is about +29.65 degrees) The path of the Sun on the equinoxes and solstices at latitude 0 degrees ( Observer at equator) Apparent Daily Motion of the Sun Solar day: 24 hours The Sun: Rises in the east Sets in the west Travels on an arc across the sky Solar and Sidereal Days Solar day (relative to the Sun): It is the average time between two consecutive passes of the Sun through the meridian. It is on average 24 hours Sidereal day (Relative to stars): It is the time between two consecutive passes of a star through the meridian. It is on average 23 hours, 56 minutes, 4.1 seconds Why is the Solar Day Longer? The reason: Earth rotation on its axis + orbital motion around the Sun The Earth has to travel an additional angle to have the Sun at the same position each day. 1 orbit = 1 full circle = 360 degrees Earth takes 1 year = 365 days to complete 1 orbit. additional angle Earth has to rotate: 360 degrees/365 days = 0.986 degrees/day How long does it take the Earth to cover ~ 1 degree? It takes 1 day to rotate 360 degrees on its axis 1 day = 24 hrs = 1440 minutes 1440 minutes/360 degrees = 4 min/degree Solar day is 4 minutes longer Apparent Annual Motion of the Sun Because of Earth orbital motion, the Sun position relative to the stars is different every night. Apparent Annual Motion of the Sun The Sun apparent path relative to the stars is called the ECLIPTIC The Sun moves eastward relative to the stars on celestial sphere It moves ~ 1 degree per day. Why? The 12 constellations through which the Sun moves are the constellations of the ZODIAC What is a constellation? A constellation is a region of the sky limited by lines of RA and Dec. The ancients attached a figure to a constellation. The IAU defined 88 constellations that cover the celestial sphere An example of a constellation: Orion the Hunter The stars form the figure of a Hunter but the stars are located a different distances. The stars in a constellation are not physical related to each other The Zodiac constellations (All the Zodiac constellations lie along the ecliptic) ZODIAC CONSTELLATIONS • There are actually 13 (NOT 12) zodiac constellations (Ophiuchus) • Sky has changed since Babylonians came up with the signs of the zodiac (Earth precession later) • For example: August 4th is not Leo anymore, but Cancer • The Sun spends different times in different constellations (they are not all the same size!) • Scorpius only 7 days • Virgo 47 days The seasons Chapter 2 Section 2.2 Why do we have seasons? Question TRUE OR FALSE? We have seasons because the Earth is closer to the Sun in summer and farther from the Sun in winter. Question TRUE OR FALSE? We have season because the Earth is closer to the Sun in summer and farther from the Sun in winter. Hint: When it is summer in America, it is winter in Australia. TRUE OR FALSE! Earth is closer to the Sun in summer and farther from the Sun in winter. Actually it is the opposite: Earth is closer to the Sun during the north hemisphere winter and farter during the north hemisphere summer (see another slide later) • Seasons are opposite in the N and S hemispheres, so distance cannot be the reason. • The real reason for seasons involves Earth’s axis tilt. What causes the seasons? Seasons depend on how Earth’s axis affects the directness of sunlight. Earth’s rotation axis is tilted by 23.5 degrees compared to the direction perpendicular to the Earth’s orbital plane 23.5 The sun crosses the meridian higher during the summer. In the winter the sun crosses the meridian lower in the sky. Seasons Summary: The Real Reason for Seasons • Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun.
Load more

Recommended publications
  • Constructing a Galactic Coordinate System Based on Near-Infrared and Radio Catalogs
    A&A 536, A102 (2011) Astronomy DOI: 10.1051/0004-6361/201116947 & c ESO 2011 Astrophysics Constructing a Galactic coordinate system based on near-infrared and radio catalogs J.-C. Liu1,2,Z.Zhu1,2, and B. Hu3,4 1 Department of astronomy, Nanjing University, Nanjing 210093, PR China e-mail: [jcliu;zhuzi]@nju.edu.cn 2 key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China 3 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China 4 Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China e-mail: [email protected] Received 24 March 2011 / Accepted 13 October 2011 ABSTRACT Context. The definition of the Galactic coordinate system was announced by the IAU Sub-Commission 33b on behalf of the IAU in 1958. An unrigorous transformation was adopted by the Hipparcos group to transform the Galactic coordinate system from the FK4-based B1950.0 system to the FK5-based J2000.0 system or to the International Celestial Reference System (ICRS). For more than 50 years, the definition of the Galactic coordinate system has remained unchanged from this IAU1958 version. On the basis of deep and all-sky catalogs, the position of the Galactic plane can be revised and updated definitions of the Galactic coordinate systems can be proposed. Aims. We re-determine the position of the Galactic plane based on modern large catalogs, such as the Two-Micron All-Sky Survey (2MASS) and the SPECFIND v2.0. This paper also aims to propose a possible definition of the optimal Galactic coordinate system by adopting the ICRS position of the Sgr A* at the Galactic center.
    [Show full text]
  • Basic Principles of Celestial Navigation James A
    Basic principles of celestial navigation James A. Van Allena) Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa 52242 ͑Received 16 January 2004; accepted 10 June 2004͒ Celestial navigation is a technique for determining one’s geographic position by the observation of identified stars, identified planets, the Sun, and the Moon. This subject has a multitude of refinements which, although valuable to a professional navigator, tend to obscure the basic principles. I describe these principles, give an analytical solution of the classical two-star-sight problem without any dependence on prior knowledge of position, and include several examples. Some approximations and simplifications are made in the interest of clarity. © 2004 American Association of Physics Teachers. ͓DOI: 10.1119/1.1778391͔ I. INTRODUCTION longitude ⌳ is between 0° and 360°, although often it is convenient to take the longitude westward of the prime me- Celestial navigation is a technique for determining one’s ridian to be between 0° and Ϫ180°. The longitude of P also geographic position by the observation of identified stars, can be specified by the plane angle in the equatorial plane identified planets, the Sun, and the Moon. Its basic principles whose vertex is at O with one radial line through the point at are a combination of rudimentary astronomical knowledge 1–3 which the meridian through P intersects the equatorial plane and spherical trigonometry. and the other radial line through the point G at which the Anyone who has been on a ship that is remote from any prime meridian intersects the equatorial plane ͑see Fig.
    [Show full text]
  • Constellations with Prominent Stars That Can Be Found Near the Meridian at 10 Pm on January 15
    ONSTELLATIONS C Altitude Ruler The rotation of the Earth on its axis causes the stars to rise and set each evening. In addition, the orbit of the Earth around the Sun places different regions of the sky in our Horizon night-time view. The PLANISPHERE is an extremely useful tool for finding stars and 10 constellation in the sky, depicting not only what is currently in the sky but it also allows the 20 prediction of the rising and setting times of various celestial objects. 30 THE LAYOUT OF THE PLANISPHERE 40 50 The outer circumference of the dark blue circular disk (which is called the star wheel) you’ll notice that the wheel is divided into the 12 months, and that each month is divided into 60 individual dates. The star wheel rotates about the brass fastener, which represents the 70 North Celestial Pole. The frame of the planisphere has times along the outer edge. 80 Holding the planisphere on the southern corner you'll see "midnight" at the top. Moving Zenith counterclockwise, notice how the hours progress, through 1 AM, 2 AM, and so on through "noon" at the bottom. The hours then proceed through the afternoon and evening (1 PM, 2 PM, etc.) back toward midnight. Once you have the wheel set properly for the correct time and day, the displayed part represents what you see if you stand with the star and planet locator held directly over your head with the brass fastener toward the north. (Notice that the compass directions are also written on the corners of the frame.) Of course, you don't have to actually stand that way to make use of the Star and Planet Locator--this is just a description to help you understand what is displayed.
    [Show full text]
  • Equatorial and Cartesian Coordinates • Consider the Unit Sphere (“Unit”: I.E
    Coordinate Transforms Equatorial and Cartesian Coordinates • Consider the unit sphere (“unit”: i.e. declination the distance from the center of the (δ) sphere to its surface is r = 1) • Then the equatorial coordinates Equator can be transformed into Cartesian coordinates: right ascension (α) – x = cos(α) cos(δ) – y = sin(α) cos(δ) z x – z = sin(δ) y • It can be much easier to use Cartesian coordinates for some manipulations of geometry in the sky Equatorial and Cartesian Coordinates • Consider the unit sphere (“unit”: i.e. the distance y x = Rcosα from the center of the y = Rsinα α R sphere to its surface is r = 1) x Right • Then the equatorial Ascension (α) coordinates can be transformed into Cartesian coordinates: declination (δ) – x = cos(α)cos(δ) z r = 1 – y = sin(α)cos(δ) δ R = rcosδ R – z = sin(δ) z = rsinδ Precession • Because the Earth is not a perfect sphere, it wobbles as it spins around its axis • This effect is known as precession • The equatorial coordinate system relies on the idea that the Earth rotates such that only Right Ascension, and not declination, is a time-dependent coordinate The effects of Precession • Currently, the star Polaris is the North Star (it lies roughly above the Earth’s North Pole at δ = 90oN) • But, over the course of about 26,000 years a variety of different points in the sky will truly be at δ = 90oN • The declination coordinate is time-dependent albeit on very long timescales • A precise astronomical coordinate system must account for this effect Equatorial coordinates and equinoxes • To account
    [Show full text]
  • Earth-Centred Universe
    Earth-centred Universe The fixed stars appear on the celestial sphere Earth rotates in one sidereal day The solar day is longer by about 4 minutes → scattered sunlight obscures the stars by day The constellations are historical → learn to recognise: Ursa Major, Ursa Minor, Cassiopeia, Pegasus, Auriga, Gemini, Orion, Taurus Sun’s Motion in the Sky The Sun moves West to East against the background of Stars Stars Stars stars Us Us Us Sun Sun Sun z z z Start 1 sidereal day later 1 solar day later Compared to the stars, the Sun takes on average 3 min 56.5 sec extra to go round once The Sun does not travel quite at a constant speed, making the actual length of a solar day vary throughout the year Pleiades Stars near the Sun Sun Above the atmosphere: stars seen near the Sun by the SOHO probe Shield Sun in Taurus Image: Hyades http://sohowww.nascom.nasa.g ov//data/realtime/javagif/gifs/20 070525_0042_c3.gif Constellations Figures courtesy: K & K From The Beauty of the Heavens by C. F. Blunt (1842) The Celestial Sphere The celestial sphere rotates anti-clockwise looking north → Its fixed points are the north celestial pole and the south celestial pole All the stars on the celestial equator are above the Earth’s equator How high in the sky is the pole star? It is as high as your latitude on the Earth Motion of the Sky (animated ) Courtesy: K & K Pole Star above the Horizon To north celestial pole Zenith The latitude of Northern horizon Aberdeen is the angle at 57º the centre of the Earth A Earth shown in the diagram as 57° 57º Equator Centre The pole star is the same angle above the northern horizon as your latitude.
    [Show full text]
  • The Sky Tonight
    MARCH POUTŪ-TE-RANGI HIGHLIGHTS Conjunction of Saturn and the Moon A conjunction is when two astronomical objects appear close in the sky as seen THE- SKY TONIGHT- - from Earth. The planets, along with the TE AHUA O TE RAKI I TENEI PO Sun and the Moon, appear to travel across Brightest Stars our sky roughly following a path called the At this time of the year, we can see the ecliptic. Each body travels at its own speed, three brightest stars in the night sky. sometimes entering ‘retrograde’ where they The brightness of a star, as seen from seem to move backwards for a period of time Earth, is measured as its apparent (though the backwards motion is only from magnitude. Pictured on the cover is our vantage point, and in fact the planets Sirius, the brightest star in our night sky, are still orbiting the Sun normally). which is 8.6 light-years away. Sometimes these celestial bodies will cross With an apparent magnitude of −1.46, paths along the ecliptic line and occupy the this star can be found in the constellation same space in our sky, though they are still Canis Major, high in the northern sky. millions of kilometres away from each other. Sirius is actually a binary star system, consisting of Sirius A which is twice the On March 19, the Moon and Saturn will be size of the Sun, and a faint white dwarf in conjunction. While the unaided eye will companion named Sirius B. only see Saturn as a bright star-like object (Saturn is the eighth brightest object in our Sirius is almost twice as bright as the night sky), a telescope can offer a spectacular second brightest star in the night sky, view of the ringed planet close to our Moon.
    [Show full text]
  • Educator's Guide: Orion
    Legends of the Night Sky Orion Educator’s Guide Grades K - 8 Written By: Dr. Phil Wymer, Ph.D. & Art Klinger Legends of the Night Sky: Orion Educator’s Guide Table of Contents Introduction………………………………………………………………....3 Constellations; General Overview……………………………………..4 Orion…………………………………………………………………………..22 Scorpius……………………………………………………………………….36 Canis Major…………………………………………………………………..45 Canis Minor…………………………………………………………………..52 Lesson Plans………………………………………………………………….56 Coloring Book…………………………………………………………………….….57 Hand Angles……………………………………………………………………….…64 Constellation Research..…………………………………………………….……71 When and Where to View Orion…………………………………….……..…77 Angles For Locating Orion..…………………………………………...……….78 Overhead Projector Punch Out of Orion……………………………………82 Where on Earth is: Thrace, Lemnos, and Crete?.............................83 Appendix………………………………………………………………………86 Copyright©2003, Audio Visual Imagineering, Inc. 2 Legends of the Night Sky: Orion Educator’s Guide Introduction It is our belief that “Legends of the Night sky: Orion” is the best multi-grade (K – 8), multi-disciplinary education package on the market today. It consists of a humorous 24-minute show and educator’s package. The Orion Educator’s Guide is designed for Planetarians, Teachers, and parents. The information is researched, organized, and laid out so that the educator need not spend hours coming up with lesson plans or labs. This has already been accomplished by certified educators. The guide is written to alleviate the fear of space and the night sky (that many elementary and middle school teachers have) when it comes to that section of the science lesson plan. It is an excellent tool that allows the parents to be a part of the learning experience. The guide is devised in such a way that there are plenty of visuals to assist the educator and student in finding the Winter constellations.
    [Show full text]
  • The Sundial Cities
    The Sundial Cities Joel Van Cranenbroeck, Belgium Keywords: Engineering survey;Implementation of plans;Positioning;Spatial planning;Urban renewal; SUMMARY When observing in our modern cities the sun shade gliding along the large surfaces of buildings and towers, an observer can notice that after all the local time could be deduced from the position of the sun. The highest building in the world - the Burj Dubai - is de facto the largest sundial ever designed. The principles of sundials can be understood most easily from an ancient model of the Sun's motion. Science has established that the Earth rotates on its axis, and revolves in an elliptic orbit about the Sun; however, meticulous astronomical observations and physics experiments were required to establish this. For navigational and sundial purposes, it is an excellent approximation to assume that the Sun revolves around a stationary Earth on the celestial sphere, which rotates every 23 hours and 56 minutes about its celestial axis, the line connecting the celestial poles. Since the celestial axis is aligned with the axis about which the Earth rotates, its angle with the local horizontal equals the local geographical latitude. Unlike the fixed stars, the Sun changes its position on the celestial sphere, being at positive declination in summer, at negative declination in winter, and having exactly zero declination (i.e., being on the celestial equator) at the equinoxes. The path of the Sun on the celestial sphere is known as the ecliptic, which passes through the twelve constellations of the zodiac in the course of a year. This model of the Sun's motion helps to understand the principles of sundials.
    [Show full text]
  • 2. Descriptive Astronomy (“Astronomy Without a Telescope”)
    2. Descriptive Astronomy (“Astronomy Without a Telescope”) http://apod.nasa.gov/apod/astropix.html • How do we locate stars in the heavens? • What stars are visible from a given location? • Where is the sun in the sky at any given time? • Where are you on the Earth? An “asterism” is two stars that appear To be close in the sky but actually aren’t In 1930 the International Astronomical Union (IAU) ruled the heavens off into 88 legal, precise constellations. (52 N, 36 S) Every star, galaxy, etc., is a member of one of these constellations. Many stars are named according to their constellation and relative brightness (Bayer 1603). Sirius α − Centauri, α-Canis declination less http://calgary.rasc.ca/constellation.htm - list than -53o not Majoris, α-Orionis visible from SC http://www.google.com/sky/ Betelgeuse https://en.wikipedia.org/wiki/List_of_Messier_objects (1758 – 1782) Biggest constellation – Hydra – the female water snake 1303 square degrees, but Ursa Major and Virgo almost as big. Hydrus – the male water snake is much smaller – 2243 square degrees Smallest is Crux – the Southern Cross – 68 square degrees Brief History Some of the current constellations can be traced back to the inhabitants of the Euphrates valley, from whom they were handed down through the Greeks and Arabs. Few pictorial records of the ancient constellation figures have survived, but in the Almagest AD 150, Ptolemy catalogued the positions of 1,022 of the brightest stars both in terms of celestial latitude and longitude, and of their places in 48 constellations. The Ptolemaic constellations left a blank area centered not on the present south pole but on a point which, because of precession, would have been the south pole c.
    [Show full text]
  • Celestial Sphere, Solar Motion, Coordinates
    Celestial Sphere, Solar Motion, Coordinates Lecture Outline -- 1 Reading: Astronomy Notes sections 3.1 through 3.5 Vocabulary terms used: celestial poles⎯points on celestial sphere directly above geographic poles. celestial equator⎯circle around the sky directly above the Earth’s equator. zenith⎯point on the celestial sphere that is always straight overhead. meridian⎯circle around the sky that goes through celestial poles and the zenith point. Separates the daytime motions of the Sun into “a.m.” and “p.m.”. solar day⎯time between successive meridian crossings of the Sun. Our clocks are based on this. ecliptic⎯the apparent yearly path of the Sun through the stars on the celestial sphere. It is the projection of the Earth’s orbit around the Sun onto the celestial sphere. vernal equinox⎯specific moment in the year (on March 21) when the Sun is directly on the celestial equator, moving north of the celestial equator. autumnal equinox⎯specific moment in the year (on September 22) when the Sun is directly on the celestial equator, moving south of the celestial equator. season⎯approximately three-month period bounded by an equinox and a solstice. solstice⎯specific moment in the year when the Sun is farthest away from the celestial equator. The summer solstice is when the Sun gets closest to zenith at noon (on June 21 for U.S.). The winter solstice is when the Sun gets closest to the horizon at noon (on December 21 for U.S.). latitude⎯used to specify position on the Earth, it is the number of degrees north or south of the Earth’s equator.
    [Show full text]
  • Astro110-01 Lecture 7 the Copernican Revolution
    Astro110-01 Lecture 7 The Copernican Revolution or the revolutionaries: Nicolas Copernicus (1473-1543) Tycho Brahe (1546-1601) Johannes Kepler (1571-1630) Galileo Galilei (1564-1642) Isaac Newton (1642-1727) who toppled Aristotle’s cosmos 2/2/09 Astro 110-01 Lecture 7 1 Recall: The Greek Geocentric Model of the heavenly spheres (around 400 BC) • Earth is a sphere that rests in the center • The Moon, Sun, and the planets each have their own spheres • The outermost sphere holds the stars • Most famous players: Aristotle and Plato 2/2/09 Aristotle Plato Astro 110-01 Lecture 7 2 But this made it difficult to explain the apparent retrograde motion of planets… Over a period of 10 weeks, Mars appears to stop, back up, then go forward again. Mars Retrograde Motion 2/2/09 Astro 110-01 Lecture 7 3 A way around the problem • Plato had decreed that in the heavens only circular motion was possible. • So, astronomers concocted the scheme of having the planets move in circles, called epicycles, that were themselves centered on other circles, called deferents • If an observation of a planet did not quite fit the existing system of deferents and epicycles, another epicycle could be added to improve the accuracy • This ancient system of astronomy was codified by the Alexandrian Greek astronomer Ptolemy (A.D. 100–170), in a book translated into Arabic and called Almagest. • Almagest remained the principal textbook of astronomy for 1400 years until Copernicus 2/2/09 Astro 110-01 Lecture 7 4 So how does the Ptolemaic model explain retrograde motion? Planets really do go backward in this model.
    [Show full text]
  • 1 the Equatorial Coordinate System
    General Astronomy (29:61) Fall 2013 Lecture 3 Notes , August 30, 2013 1 The Equatorial Coordinate System We can define a coordinate system fixed with respect to the stars. Just like we can specify the latitude and longitude of a place on Earth, we can specify the coordinates of a star relative to a coordinate system fixed with respect to the stars. Look at Figure 1.5 of the textbook for a definition of this coordinate system. The Equatorial Coordinate System is similar in concept to longitude and latitude. • Right Ascension ! longitude. The symbol for Right Ascension is α. The units of Right Ascension are hours, minutes, and seconds, just like time • Declination ! latitude. The symbol for Declination is δ. Declination = 0◦ cor- responds to the Celestial Equator, δ = 90◦ corresponds to the North Celestial Pole. Let's look at the Equatorial Coordinates of some objects you should have seen last night. • Arcturus: RA= 14h16m, Dec= +19◦110 (see Appendix A) • Vega: RA= 18h37m, Dec= +38◦470 (see Appendix A) • Venus: RA= 13h02m, Dec= −6◦370 • Saturn: RA= 14h21m, Dec= −11◦410 −! Hand out SC1 charts. Find these objects on them. Now find the constellation of Orion, and read off the Right Ascension and Decli- nation of the middle star in the belt. Next week in lab, you will have the chance to use the computer program Stellar- ium to display the sky and find coordinates of objects (stars, planets). 1.1 Further Remarks on the Equatorial Coordinate System The Equatorial Coordinate System is fundamentally established by the rotation axis of the Earth.
    [Show full text]