Field of View: The area visible in an image, usually given as the diameter of the region.
Scientific notation: The system of recording very large or very small numbers by using powers
10^1 = 10, 10^2 = 10 x 10 = 100
10^3 = 10 x 10 x 10 = 1000, etc.
Astronomical Unit: Average distance from Earth to the Sun; 1.5 x 10^8 kilometers
Solar System: The sun and its planets, asteroids and so on
Planet: A non-luminous body in orbit around a star, large enough to be spherical and to have
cleared its orbital zone of other objects.
Star: A globe of gas held together by its own gravity and supported by the internal pressure of
its hot-gases, which generate energy by nuclear fusion. The sun is about 110 times larger in
diameter than Earth.
Light year (ly): Unit of distance equal to the distance light travels in one year (roughly 10^13
km or 63000 AU) (Star nearest the sun = Proxima Centauri. It is 4.2 ly from Earth. Light from
Proxima Centauri takes 4.2 years to reach Earth)
Light Travel Time
• Electromagnetic waves travel at the speed of light (denoted by ‘c’ in equations) – the
speed limit of the Universe
• It takes about 1.25 seconds for Earth communications to reach the Moon
• If you’re driving 100 km/h on the 401, it takes about 2 hours to drive 200 km
• Suppose you’re driving 300 000 km/s, how far would you get in 1 second? 300 000 km
Galaxy: A large system of stars, star clusters, gas, dust, and nebulae orbiting a common centre
Milky Way: The hazy band of light circles our sky, produced by the glow of our galaxy.
Milky Way Galaxy: The spiral galaxy containing our Sun, visible in the night sky as the Milky
Way. Our galaxy contains over 100 Billion stars. The visible disk of our galaxy is roughly 80 000
ly in diameter.
Spiral arms: Long spiral pattern of bright stars, star clusters, gas, and dust. Spiral arms extend
from the centre to the edge of the disk of spiral galaxies.
Superclusters: A cluster of galaxy clusters Cosmic Calendar
1 month -> 1.2 billion years 1 day -> 40 million years 1 second -> 440 years
• The Egyptian pyramids were built about 11 seconds ago.
• Copernicus and others convinced humanity that the Earth orbits the Sun about one second
• Elizabeth II became Queen of England about 0.14 seconds ago.
• You were born about 0.04 seconds ago (assuming your age is 18).
Constellation: One of the stellar patterns identified by name, usually of mythological gods,
people, animals, or objects. Also, the region of the sky containing that star pattern. Names
originated in ancient civilizations beginning as much as 5000 years ago. 48 are still in use. In
former times, constellations were simply loose groupings of bright stars and many of the fainter
stars were not included. Now, a constellation now represents a section of the sky (viewing
direction) and any star within the region belongs to that constellation.
Asterism:Anamed grouping of stars that is not one of the recognized constellations (eg. The
Big Dipper that is part of the Ursa Major constellation).
• Although constellations and asterisms are named as if they are real groupings, most are
made up of stars that are not physically associated with one another. Some stars may be
many times further away than others in the same constellation and moving through space
in different directions. The only thing in common may be the direction they are moving
as viewed from Earth. Names of Stars
• Brightest stars – alpha
• Second brightest – beta
Brightness of Stars
Magnitude scale: The astronomical brightness scale. The larger the number, the fainter the
Hipparchus is believed to have created the first star catalogue, there is evidence he used the
300 years later, Ptolemy used the magnitude system in his own catalogue, and successive
generations of astronomers have continued to use the system.
Apparent visual magnitude: A measure of the brightness of a star as seen by human eyes on
Earth. Brightness is quite subjective, depending on the physiology of eyes and the psychology
Flux: A measure of the flow of energy out of a surface. Usually applied to light. A measure of the
light energy from a star that hits one square metre in one second. With modern scientific
instruments, astronomers can measure the flux of starlight with high precision and then use a
simple mathematical relationship that relates light flux to apparent visual magnitude.
Limitations of the apparent visual magnitude system have motivated astronomers to supplement
it in various ways:
• Some stars are so bright the magnitude scale must extend into negative numbers.
• With a telescope, you can find stars much fainter than the limit for your unaided eyes.
Thus the magnitude system has also been extended to numbers larger than sixth
magnitude to include faint stars.
• Although some stars emit large amounts of infrared or ultraviolet light, those types of
radiation are invisible to human eyes. Other magnitude systems have been invented to
express the brightness of invisible light arriving at Earth from the stars.
• An apparent magnitude tells only how bright a star is as seen from Earth but doesn’t tell
anything about the star’s true power output because the star’s distance is not included.
The Sky and its Motions
Celestial sphere: An imaginary sphere of very large radius surrounding Earth to which planets,
stars, Sun and Moon seem to be attached. Modern astronomers know that the stars are
scattered through space at different distances, but it is still convenient and useful in some
Scientific model: A concept that helps you think about some aspect of nature without
necessarily being true. • Sky objects rotate westward around Earth each day, but that is a consequence of the
eastern rotation of Earth. This produces day and night.
• What you can see in the sky depends on where you are on Earth.
• Astronomers measure distances across the sky at angles expressed in units of degrees and
subdivisions of degrees called arc minutes and arc seconds.
Precession: The slow change in orientation of the Earth’s axis of rotation. One cycle takes
nearly 26 000 years.
• Earth’s rapid rotation keep its rotation axis pointed towards a spot near the star Polaris. If
Earth were a perfect sphere, the axis direction would not move. Earth has a slight bulge
around its middle which the gravity of the Sun and moon pull on, tending to twist the
Earth’s axis. This process causes Earth’s axis to precess in a slow circular sweep. It takes
about 26 000 years for the axis to complete one cycle.
The Sky around Us
Horizon – From any location on Earth, you can only see half of the celestial sphere, the half
above the horizon line, the circular boundary between Earth and sky.
Zenith – marks the top of the sky above your head
Nadir – marks the bottom of the sky directly under your feet
North celestial pole and south celestial pole – located directly above Earth’s north and south
poles, the apparent pivot points.
Celestial equator – The imaginary line in the sky directly above Earth’s equator
Circumpolar constellations – constellations that never rise or set
The Annual Motion of the Sun
Rotation: Motion around an axis passing through the rotating body.
Revolution: Orbital motion about a point located outside the orbiting body.
• Earth rotates on an axis which produces day and night but it also revolves around the sun
which produces the yearly cycle.
• The daytime sky is actually still filled with stars but the glare from the sun fills the
atmosphere with scattered light, causing us to perceive the sky as brilliant blue.
• If the Sun were fainter: We could see stars in the daytime as well as observe the sun
moving eastward across the sky relative to the background of stars. This apparent motion
is caused by the orbital motion of the Earth around the sun.
• January = the Sun would be in front of the constellation Sagittarius
• March = the Sun would be in front of the constellationAquarius
• Through the year, the Sun moves eastward, following a line known as the ecliptic, the
apparent path of the Sun around the sky. • Earth makes a full circle around the Sun in 365.26 days. Thus, the Sun appears to go
around the sky in the same amount of time. We don’t notice this motion because we
cannot see the stars in daylight.
• Seasons are caused by the revolution of Earth around the Sun combined with the Earth’s
axial tilt of 23.5 degrees relative to its orbit.
• Misconception: Seasons are not caused by variations in the distance between Earth and
the Sun (Earth’s Orbit is nearly circular and is always about the same distance from the
• Seasons are changed by changes in the amount of solar energy that the