• Day ↔ Night

» the sky: stars, planets, moon, ...

• 2000 - 3000 stars are visible to the naked eye during the night.

• People of nearly every culture gave names to patterns in the sky:

  • the patterns of stars seen in the sky → constellations

» Early Astronomers used constellations for navigational purposes -or- for calendars to predict planting/harvesting

» Today's Astronomers however use the constellations to refer to a large area of the sky (like continents)

• During the night the constellations seem to move smoothly across the sky from east to west.

» ... but relative locations of stars remain unchanged as the sky move

» ... the stars must be firmly attached to a Celestial Sphere surrounding the Earth (see the picture on the right).

• Modern Standpoint: apparent motion of stars is the result of the spin (rotation - not of the celestial sphere but of the earth)

» ... Even though Celestial Sphere is an incorrect description we still use the idea as a convenient fiction to visualize the position of the stars in the sky.

• Illusion: The stars in a particular constellation may appear to lie close to one another

» ... but they may be quite far apart in reality because they may lie at very different distances from Earth.

» ... due to lack of perception when we look into space

» ... we should be concluding that the stars are so far away

The Local Sky

• Stars rise and set - like the sun, moon and planets ...

» In reality, we are the ones who are moving.

• Objects appear to rise in the east and set in the west

» ... because Earth rotates in the opposite direction: west → east

• If you could view the celestial sphere from outside,

» ... the daily motion of the stars would appear as circles

• Since we live on Earth, we see only half of the celestial sphere.

  • It is called the local sky.

  • Two angle measurements are required to locate any object in the local sky.

  • Each angle has a zero point.

    1. Azimuth (direction) - Starts from North, measured along East, South, West and back to North

    2. Altitude (elevation) - Starts from the horizon rises up to Zenith (direction where your head is pointing).

Angle Measurements

• Due to lack of perception in the sky we cannot tell the true sizes of objects or the true distances between objects just by looking at them.

» ... Instead we measure angles in the sky:

• Angles are measured in sexadecimal units:

  • 1 complete circle = 360 degrees (2π radians)

  • 1 degree = 60 arcminutes

  • 1 arcminute = 60 arcseconds 

Geographical Coordinate System

• The local sky can be stretched to the whole Earth surface.

» ... creating a new coordinate system: geographical 

» Similarly, a location on Earth has to be represented with two angle measurements:

• 1. Longitude (along the rotation direction of Earth) - Starts from Greenwich, plus values towards East, minus values towards West.

  2. Latitude (subtended from Equator) - Starts from Equator, plus values towards North, minus values towards South.

Celestial Coordinate System

• The local sky can be extended further until it reaches to the celestial sphere which has a radius of infinity.

• When the local sky is overlayed to the celestial sphere

» ... motion of objects in the sky can be observed.

» ... creating paths on the celestial sphere.

• • Circumpolar stars never rise or set but instead make daily counterclockwise circles around NCP

  • Non-circumpolar stars never rise and remain constantly below the horizon.

  • The rest of the stars daily rise in the east and set in the west.

  • The paths of the Sun, Moon and planets follow these rules as well.

• From the geometry and by observing the local sky, Geographic Location on Earth can be calculated:

» Altitude of the Celestial Pole = Your Geographic Latitude 

• Similarly each object on the celestial sphere can be identified from two angles.

  1. Right Ascension

  2. Declination

See Celestial Coordinate System ⤤ for more details. 

Annual Changes

• As we orbit the Sun over the course of a year it appears to move against the background stars in the constellations.

• If we could see both the Sun and the stars we would notice the Sun gradually moving eastward along the ecliptic.

• The constellations along the ecliptic are called constellations of the Zodiac.

• Nearest object to Earth and second bright in the sky

• We observe two types of change in the Moon's motion in the sky.

  • Appearance

  • Rise and set times

» ... these changes lead to → Lunar Phases

• Distinctive Phases:

  • New Moon: not visible (The Moon cannot be distinguished from Sun's light)

  • First and Third Quarters: The angle between Sun - Earth - Moon is 90 degrees

  • Full Moon: The disk of the Moon is fully illuminated by the Sun.

Eclipses

Definition of an eclipse: Any time one astronomical object casts a shadow on another.

Due to the geometry of lunar phases one would expect the followings: 

• A new moon always blocks our view of the Sun.

• Earth would always prevent sunlight from reaching a full Moon.

• So, why there is not always a solar/lunar eclipse every new/full moon?

» ... Moon's orbit is inclined to the ecliptic plane by ~ 5 degrees.

Definition of Nodes: The two points in each orbit (Moon's around the Earth and Earth's around the Sun) at which the Moon crosses the ecliptic plane.

• The nodes of the Moon's orbit must be nearly aligned with the Sun and Earth.

• The phase of the Moon at this alignment must be either new or full.

Predictions: The times when the lines of nodes are directed towards the sun are favorable.

» ... eclipse seasons ~ 18 years, 11 1/5 days → Saros Cycle