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Astronomy 101. 1. Where we are in the Universe. 2. Motions on the sky. Milky Way Galaxy. 25,000 light years, Or ~ 8 kpc. 200 billion stars. Galactic year = 225 million yr Our sun is 4.6 billion yr old. 1 pc = 3.26 ly. The parallax angle p. Small-angle formula:.

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Astronomy 101

1. Where we are in the Universe

2. Motions on the sky


Milky Way Galaxy

25,000 light years,

Or ~ 8 kpc

200 billion stars

Galactic year = 225 million yr

Our sun is 4.6 billion yr old

1 pc = 3.26 ly


The parallax angle p

Small-angle formula:

Define 1 parsec as a distance to a star whose parallax is 1 arcsec

d (in parsecs) = 1/p

1 pc = 206265 AU = 3.26 ly


Hubble Deep Field

10 day exposure photo!

Over 1500 galaxies in a spot 1/30 the diameter of the Moon

Farthest and oldest objects are 13 billion light years away!

Hubble Space


~ 100 billion galaxies in the observable Universe

the galactic center
The Galactic Center

Our view (in visible light) towards the galactic center (GC) is heavily obscured by gas and dust

Extinction by 30 magnitudes

 Only 1 out of 1012 optical photons makes its way from the GC towards Earth!

Galactic center

Wide-angle optical view of the GC region


If one looks at this region with big telescopes and near-infrared cameras one can see lots of stars. If one takes pictures every year it seems that some stars are moving very fast (up to 1500 kilometers per second). The fastest stars are in the very center - the position marked by the radio nucleus Sagittarius A* (cross).

Distance between stars is less that 0.01 pc

a black hole at the center of our galaxy
A Black Hole at the Center of Our Galaxy?

By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to ~ 2.6 million solar masses


Radio observations with Very Long Baseline Interferometry (VLBI) that are thousands of times more precise than optical observations (good enough to easily pin-point a source the size of a pea in New York when sitting in Paris)


Recent VLBI observations (latest issue of Nature)

Size ~ 1 AU (12 Schwarzschild Radii)

Density ~ 7x1021 Msun/pc3


The Kuiper Belt – home for short-period comets??

Starting in 1992, astronomers have become aware of a vast population of small bodies orbiting the sun beyond Neptune. There are at least 70,000 "trans-Neptunians" with diameters larger than 100 km in the radial zone extending outwards from the orbit of Neptune (at 30 AU) to 50 AU.


Voyagers 1 and 2

Launched in 1977

Voyager 1 is now 95 AU from the Sun!

(13 light-hours, or 14 billion km)

The most distant human-made object in the Universe

Speed 17.2 km/sec (3.6 AU per year)


Proxima Centauri (Alpha Centauri C)

Closest star (4.2 light-years from the Sun)

It would take ~ 80,000 years for Voyager 1 to reach a neighboring star

Plutonium battery will be dead by 2020

Mission may be shut down by 11/2005

Golden record


Local Bubble

Density ~ 0.05 atoms/cm3

Temperature ~ 105 K

Remnant of supernova



1017 m

= 3 pc




107 m


109 m


1011 m

= 1 AU

Solar System

1021 m

= 10 kpc


1025 m

= 100 Mpc



1026 m

= Gpc



Distance scale

Looking through space = travel in time!

Classification of objects on the sky
  • Description of motions of these objects
  • Understanding 1 and 2

The constellations are an ancient heritage handed down for thousands of years as celebrations of great heroes and mythical creatures. Here Sagittarius and Scorpius hang above the southern horizon.


In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures.

constellations 2
Constellations (2)

Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.


International Astronomical Union (IAU)

Names and Standard Abbreviations of Constellations

The following list of constellation names and abbreviations is in accordance with the resolutions of the International Astronomical Union (Trans. IAU, 1, 158; 4, 221; 9, 66 and 77). The boundaries of the constellations are listed by E. Delporte, on behalf of the IAU, in, Delimitation scientifique des constellations (tables et cartes), Cambridge University Press, 1930; they lie along the meridians of right ascension and paralleIs of declination for the mean equator and equinox of 1875.0.

88 constellations


Hipparchus of Rhodes

Born: 190 BC in Nicaea (now Iznik), Bithynia (now Turkey)Died: 120 BC in probably Rhodes, Greece

Catalogue of 850 stars

Discovered precession of the Earth’s orbit

Determined the distance to the moon

Compiled trigonometric tables

For thousands of years, discoveries in math and science were

driven by astronomical observations!


Claudius Ptolemy

Born: about 85 in EgyptDied: about 165 in Alexandria, Egypt


  • A treatise in 13 books
  • Mathematical theory of the motions
  • of the Sun, moon, and planets
  • Catalogue of 1022 stars and 48 constellations
  • Introduced minutes and seconds
  • Geocentric system

Shares with Euclid's "Elements" the glory of being the scientific text longest in use.


Original book title is Syntaxis

  • Translated to Arabic as Almagest (al majisti) and then to Latin
  • That is why stars have Arabic names

Venice: Petrus Liechtenstein, 1515.



Bright stars from first to third magnitude have proper names that have been in use for hundreds of years. Most of these names are Arabic. Examples are Betelgeuse, the bright orange star in the constellation Orion, and Dubhe, the second-magnitude star at the edge of the Big Dipper's cup (Ursa Major). A few proper star names are not Arabic. One is Polaris, the second-magnitude star at the end of the handle of the Little Dipper (Ursa Minor). Polaris also carries the popular name, the North Star. A second system for naming bright stars was introduced in 1603 by J. Bayer of Bavaria. In his constellation atlas, Bayer assigned successive letters of the Greek alphabet to the brighter stars of each constellation. Each Bayer designation is the Greek letter with the genitive form of the constellation name. Thus Polaris is Alpha Ursae Minoris. Occasionally, Bayer switched brightness order for serial order in assigning Greek letters. An example of this is Dubhe as Alpha Ursae Majoris, with each star along the Big Dipper from the cup to handle having the next Greek letter. Faint stars are designated in different ways in catalogs prepared and used by astronomers. One is the Bonner Durchmusterung, compiled at Bonn Observatory starting in 1837. A third of a million stars are listed by "BD numbers." The Smithsonian Astrophysical Observatory (SAO) Catalogue, the Yale Star Catalog, and The Henry Draper Catalog published by Harvard College Observatory are all widely used by astronomers. The Supernova of 1987 (Supernova 1987a), one of the major astronomical events of this century, was identified with the star named SK -69 202 in the very specialized catalog, the Deep Objective Prism Survey of the Large Magellanic Cloud, published by the Warner and Swasey Observatory. These procedures and catalogs accepted by the International Astronomical Union are the only means by which stars receive long-lasting names.


The celestial sphere

The entire sky appears to turn around imaginary points in the northern and southern sky once in 24 hours. This is termed the daily or diurnal motion of the celestial sphere, and is in reality a consequence of the daily rotation of the earth on its axis. The diurnal motion affects all objects in the sky and does not change their relative positions: the diurnal motion causes the sky to rotate as a whole once every 24 hours.

Superposed on the overall diurnal motion of the sky is "intrinsic" motion that causes certain objects on the celestial sphere to change their positions with respect to the other objects on the celestial sphere. These are the "wanderers" of the ancient astronomers: the planets, the Sun, and the Moon.


We can define a useful coordinate system for locating objects on the celestial sphere by projecting onto the sky the latitude-longitude coordinate system that we use on the surface of the earth.

The stars rotate around the North and South Celestial Poles. These are the points in the sky directly above the geographic north and south pole, respectively. The Earth's axis of rotation intersects the celestial sphere at the celestial poles. Fortunately, for those in the northern hemisphere, there is a fairly bright star real close to the North Celestial Pole (Polaris or the North star). Another important reference marker is the celestial equator: an imaginary circle around the sky directly above the Earth's equator. It is always 90 degrees from the poles. All the stars rotate in a path that is parallel to the celestial equator. The celestial equator intercepts the horizon at the points directly east and west anywhere on the Earth.

the celestial sphere 2
The Celestial Sphere (2)
  • From geographic latitude L (northern hemisphere), you see the celestial north pole L degrees above the horizon;
  • From geographic latitude –L (southern hemisphere), you see the celestial south pole Ldegrees above the horizon.

90o - L

  • Celestial equator culminates 90º – L above the horizon.



Equatorial coordinates

Declination (similar to latitude)

Right ascension (similar to longitude)

Counted from celestial equator

Measured in degrees etc.

Counted from Vernal Equinox

Measured in hours, minutes, seconds

Full circle is 24 hours


The arc that goes through the north point on the horizon, zenith, and south point on the horizon is called the meridian. The positions of the zenith and meridian with respect to the stars will change as the celestial sphere rotates and if the observer changes locations on the Earth, but those reference marks do not change with respect to the observer's horizon. Any celestial object crossing the meridian is at its highest altitude (distance from the horizon) during that night (or day).

During daylight, the meridian separates the morning and afternoon positions of the Sun. In the morning the Sun is ``ante meridiem'' (Latin for ``before meridian'') or east of the meridian, abbreviated ``a.m.''. At local noon the Sun is right on the meridian. At local noon the Sun is due south for northern hemisphere observers and due north for southern hemisphere observers. In the afternoon the Sun is ``post meridiem'' (Latin for ``after meridian'') or west of the meridian, abbreviated ``p.m.''.


If you are in the northern hemisphere, celestial objects north of the celestial equator are above the horizon for more than 12 hours because you see more than half of their total 24-hour path around you. Celestial objects on the celestial equator are up 12 hours and those south of the celestial equator are above the horizon for less than 12 hours because you see less than half of their total 24-hour path around you. The opposite is true if you are in the southern hemisphere.

Notice that stars closer to the NCP are above the horizon longer than those farther away from the NCP. Those stars within an angular distance from the NCP equal to the observer's latitude are above the horizon for 24 hours---they are circumpolar stars. Also, those stars close enough to the SCP (within a distance = observer's latitude) will never rise above the horizon. They are also called circumpolar stars.

precession 1
Precession (1)

At left, gravity is pulling on a slanted top. => Wobbling around the vertical.

The Sun’s gravity is doing the same to Earth.

The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.

precession 2
Precession (2)

As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years.

It will be closest to Polaris ~ A.D. 2100.

There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.)

~ 12,000 years from now, it will be close to Vega in the constellation Lyra.

the sun and its motions
The Sun and Its Motions

Earth’s rotation is causing the day/night cycle.


The "Road of the Sun" on the Celestial Sphere

  • Diurnal motion from east to west due to the earth’s spinning around its axis, with ~ 24 h period
  • Drift eastward with respect to the stars ~ 1 degree per day with the period of ~ 365.25 days.
  • This causes the difference of 4 min per day between the Solar and Sidereal day.
the ecliptic
The Ecliptic

Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations.

The Sun’s apparent path on the sky is called the Ecliptic.

Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere.

Sun travels 360o/365.25 days ~ 1o/day

the seasons
The Seasons

Earth’s axis of rotation is inclined vs. the normal to its orbital plane by 23.5°, which causes the seasons.


We experience Summer in the Northern Hemisphere when the Earth is on that part of its orbit where the N. Hemisphere is oriented more toward the Sun and therefore:

  • the Sun rises higher in the sky and is above the horizon longer,
  • The rays of the Sun strike the ground more directly.
  • Likewise, in the N. Hemisphere Winter the hemisphere is oriented away from the Sun, the Sun only rises low in the sky, is above the horizon for a shorter period, and the rays of the Sun strike the ground more obliquely.

Seasons are NOT caused by varying distances from the Earth to the Sun

  • The primary cause of seasons is the 23.5 degree tilt of the
  • Earth's rotation axis with respect to the plane of the ecliptic.

Note: the Earth is actually closest to the Sun in January 4!

Perihelion: 147.09 × 106 km; Aphelion: 152.10 × 106 km


The ecliptic and celestial equator intersect at two points: the vernal (spring) equinox and autumnal (fall) equinox. The Sun crosses the celestial equator moving northward at the vernal equinox around March 21 and crosses the celestial equator moving southward at the autumnal equinox around September 22.

When the Sun is on the celestial equator at the equinoxes, everybody on the Earth experiences 12 hours of daylight and 12 hours of night for those two days (hence, the name ``equinox'' for ``equal night'').

The day of the vernal equinox marks the beginning of the three-month season of spring on our calendar and the day of the autumn equinox marks the beginning of the season of autumn (fall) on our calendar. On those two days of the year, the Sun will rise in the exact east direction, follow an arc right along the celestial equator and set in the exact west direction.


Since the ecliptic is tilted 23.5 degrees with respect to the celestial equator, the Sun's maximum angular distance from the celestial equator is 23.5 degrees. This happens at the solstices. For observers in the northern hemisphere, the farthest northern point above the celestial equator is the summer solstice, and the farthest southern point is the winter solstice. The word ``solstice'' means ``sun standing still'' because the Sun stops moving northward or southward at those points on the ecliptic.

The Sun reaches winter solstice around December 21 and you see the least part of its diurnal path all year---this is the day of the least amount of daylight and marks the beginning of the season of winter for the northern hemisphere. On that day the Sun rises at its furthest south position in the southeast, follows its lowest arc south of the celestial equator, and sets at its furthest south position in the southwest.

The Sun reaches the summer solstice around June 21 and you see the greatest part of its diurnal path above the horizon all year---this is the day of the most amount of daylight and marks the beginning of the season of summer for the northern hemisphere. On that day the Sun rises at its furthest north position in the northeast, follows its highest arc north of the celestial equator, and sets at its furthest north position in the northwest.

There are no seasons on the equator (except for the changes related to weather)
  • In reality the seasons “lag”: for example, maximum summer temperatures occur ~ 1 month later than the summer solstice. Blame oceans that act as storages of heat!

Myr ago

Puzzle: Ice Ages!

  • Occur with a period of ~ 250 million yr
  • Cycles of glaciation within the ice age occur with a period of 40,000 yr
  • Most recent ice age began ~ 3 million yr ago and is still going on!

Last Glacial Maximum: 18,000 yr ago

32% of land covered with ice

Sea level 120 m lower than now

ice age cause
Ice Age: Cause
  • Theory: climate changes due to tiny variations in the Earth’s orbital parameters
    • Precession of the rotation axis (26,000 yr cycle)
    • Eccentricity (varies from 0.00 to 0.06 with 100,000 and 400,000 yr cycles)
    • Axis tilt (varies from 24.5o to 21.5o with 41,000 yr cycle

Milutin Milankovitch 1920


Varies from 0.00 to 0.06 (currently 0.017)

  • Periodicity 100,000 and 400,000 yr
  • Eccentricity cycle modulates the amplitude of the precession cycle

Our Earth makes a complicated motion through space , like a crazy spaceship

As a result, the flux of solar radiation received by the Earth oscillates with different periodicities and amplitudes

This triggers changes in climate




f1 = sin[2 t + 1]

f2 = 0.7 sin[3.1 t + 2.4]

f3 = 1.3 sin[4.5 t + 0.3]

Adding oscillations with different phases and incommensurate frequencies





Adding Milankovitch cycles of solar irradiation for 65 degree North latitude

Note the last peak 9,000 years ago when the last large ice sheet melted

(Berger 1991)

are these effects enough to explain the ice ages
Are these effects enough to explain the Ice Ages???

Other factors? Volcanic winters, impacts, …

71,000 yr ago: eruption of Mount Toba (Sumatra)

2,800 km3 of material thrown in the atmosphere

Instant ice age?

Meteorite impacts; Mass extinctions


150 known impact sites on Earth

Diameters from 50-70 m to 200 km


Barringer crater, Arizona

49,000 yr old

Iron meteorite of size 50 m, mass 300,000 ton

Impact velocity 11 km/sec


World-wide fires

  • 1-km-hign tsunamis
  • Acid rains and atmospheric pollution
  • Darkness and severe winter for many decades

¾ of all living species have been killed


The Peekskill meteorite

October 9, 1992

12 kg stony meteorite hit the Earth