-- PH223 Astronomy A -- Abell's Chapter I.

The Scale of the Universe

Distance (length) scale

The basic unit of length is the metre (m) originally defined to be 1/10,000,000 of the distance from the equator to the north pole through Paris. For ease of comparison with a secondary standard, a standard meter was created which consists of two scratches on a length of platinum-iridium alloy. Later, the metre was defined to be 1,650,763.73 waves in vacuum of the orange resonance line (2p10-5d5) of krypton-86. The metre is now defined to be the distance light travels in exactly 1/299792458 of a second in vacuum and the second is defined to be the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels (F=4, F=3) of the ground state of the cesium atom which only has a single stable isotope, Cs-133.
The ephemeris second is defined to be 1/31,556,925.97474 of the length of the day beginning at the first of the year 1900.

The distance between Brisbane and Canberra is almost one million metres, which we write as one megametre, 10⁶m, or one thousand kilometres, 10³km. From the original definition it is easy to show the earth's radius is about 6378km. The average distance to the sun is 149,597,870.7km, and is what astronomers refer to as the astronomical unit or AU. It is common practice to measure distances within the solar system in AU, planetary sizes in earth radii and the size of smaller bodies in km. The nearest star is about 270,000AU distant and the solar system is taken to be space within 50,000AU from the sun. (It is thought there is a swarm of icy debris at the edge of the solar system, the Oort cloud, from which comets derive.)

You should be able to tell someone relative distances to the planets, size of the solar system, sizes of planets, major moons, comets, asteroids, meteoroids. Know the AU in km. A summary of these data will be provided. Beware of confusing the size of a body with its size(!) We like to say "the moon is a quarter the size of the earth". This is true, but deceptive. One should always think in terms of volume when talking size... For earth/moon that is one quarter cubed or 64 moons would fit into the earth. [actually (Re/Rm)³=(3.67)³=49.4] Mass goes like volume (times density) so when you look at the mass ratio you find the moon mass is a mere 1/81 that of the earth. If the moon's mass were added to the earth you would hardly know it.

Other astronomical distance units:

The light year (ly) is the distance light travels in one year (365.2425d). One light year amounts to 63,240 AU or 9.4605 times 10¹² km.

The parsec (pc) is preferred by astronomers over the light year for large distances. It is the distance that corresponds to a parallax of one second of arc (arcsec) with a baseline of 1AU. This is just the cosecant of one arcsec or about 206265 AU.

Early Astronomy -- PH223 Astronomy A -- Abell's Chapter 2.

The earliest practical application of astronomy was timr snf the calendar. The calendar is tricky because there is not an integral number of days in a tropical year. Add to this the fact that the moon "had" to be included in the calendar and we find the calendar is tricky indeed. Perhaps the first recorded evidence of a calendar is preserved in the various woodhenges and stonehenges culminating in the Stonehenge monument near Salisbury England. Stonehenge seems to be an early computing device that made predictions relating to the first day of spring and eclipse phenomena. Early Greek Astronomy

By 600 BC the Greek philosophers had developed a cosmology that included some definitions still in use today:

The Celestial Sphere is an imaginary "crystalline sphere" with stars imbedded. The sphere was known to be very large since there was no evidence for parallax. The sphere turns on its axis once every 23h56m so stars appear to rise four minutes earlier each day. Parallax is a surveying tool. Binocular vision exploits parallax where the interocular distance (about 65mm) is the baseline. For small angles the parallax formula can be written baseline s equals distance r times parallax angle q or s=rq. Note that q must be measured in radians. There are 206265 arcsec in a radian so if you measure q in arcsec be sure to divide by 206265. (sorry, q is really theta in the notes...)

The Celestial Poles are the points on the celestial sphere where the axis passes through the sphere, the north celestial pole and the south celestial pole. You should think about how the poles appear to observers at different latitudes and longitudes and at different times of day.

The Celestial Equator is just like the equator on a terrestrial globe and in fact is the projection of the earth's equator on the celestial sphere. Orion's belt is near the CE. So is Saturn right now. The equator "rises" due east and "sets" due west. You should be able to tell how high the equator "rises" at a given latitude.

The local meridian is not actually on the sphere but is a projection on the sphere of your meridian. It always "rises" due north, passes directly overhead (the zenith) and "sets" due south.

The Ecliptic is the path of the sun on the celestial sphere. The sun travels eastward about one degree per day on this great circle that is inclined 23.5^o to the equator. In the course of one sidereal year the sun moves once around the starry sphere. The ancients knew of this path in the sky and knew that eclipses of the sun or moon would only occur when the New or full moon happened when the moon was crossing the ecliptic. Hence, the name! (The moon travels a great circle that is inclined 5^o to the ecliptic but I don't know of any name for this path.)

Constellations are artificial divisions on the celestial sphere. There are 88 constellations and you will get to know several of them (Crux, Centaurus, Circinus, Musca and more). Get to know the nomenclature: three letters either Cap small small or Cap Cap small depending on whether constellation is single or double word. Identify the following and look for them tonight: CMa, CMi, Ori, Tau, Gem and of course Cru is to the east.

The Zodiac is a series of twelve constellations (thirteen if you include Oph which is pushing Sco out of the way) that the ecliptic passes through. The sun, moon, planets and many asteroids are found in this belt about 18^o wide centred on the ecliptic.