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Discovering The Universe for Yourself. Patterns in the Sky. The Constellation Orion. Constellations: Recognizable patterns in the sky are called Constellations. There are 88 Constellations. 12 are based on the Zodiac. Orion, Leo, Aries, Taurus, etc.

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patterns in the sky
Patterns in the Sky

The Constellation Orion

Constellations: Recognizable patterns in the sky are called Constellations.
  • There are 88 Constellations. 12 are based on the Zodiac.
    • Orion, Leo, Aries, Taurus, etc.
  • Asterisms: Recognizable patterns of stars that are not one of the constellations are called Asterisms:
    • Big Dipper, Little Dipper, Summer Triangle
Are stars still present in the sky during the day time?
    • Yes, we just can’t see them because their dim light is overwhelmed by the brightness of the sun during the day.
the milky way
The band of light called the “Milky Way” traces the galactic plane as it appears from our location in the outskirts of the galaxy. The Milky Way


© Jerry Schad

the circling sky
The Circling Sky
  • Earth rotates from west to east.
  • The celestial sphere appears to rotate east to west.

Apparent path of a particular star

angular sizes
Angular Sizes
  • We can measure angles in the sky
  • The angular size of an object like the Sun or the Moon is the angle it appears to span in your field of view.
  • The angular distance between a pair of objects is the angle that appears to separate them.

To Polaris

Angular size of moon ~ ½ o

Angular distance between the “pointer stars” ~ 5o

Latitude : Measures positions north or south of the equator. (Note: This is an angle)
    • 0o = the equator.
    • 90o N= the north pole.
    • 90o S = the south pole.
  • Longitude : Measures positions east or west of the Prime Meridian. (This is an angle)
  • The Prime meridian is an imaginary line that passes through Greenwhich, England.
In the Northern Hemisphere, stars near the North Celestial Pole are always above the horizon. Therefore, these stars are called Circumpolar Stars
  • In the Northern Hemisphere, stars near the south celestial pole remain below the horizon and so are never seen by a northern observer.
  • In the Southern Hemisphere, stars near the north celestial pole are never visible to the southern observer.

Counter-Clockwise Rotation

Clockwise Rotation

Northern Hemisphere

Southern Hemisphere

The Altitude of the celestial pole (Polaris) = your latitude

altitude and latitude
Altitude and Latitude
  • For Nacogdoches, Texas:
    • Latitude: 31o 45’ 35.3” = 31.7598o
    • Longitude: -94o 39’ 40.5” = -94.6613o
  • Question: What is the Altitude of Polaris for Nacogdoches?
  • Answer: 31o 45’ 35.3” – the same as our Latitude!
seasonal changes in the sky
Seasonal Changes in the Sky
  • The night-time constellations change with the seasons.
  • This is due to the Earth’s orbit around the Sun.
the sun s path through the daytime sky changes with seasons
The Sun’s Path Through The Daytime Sky Changes With Seasons
  • The sun traces out a characteristic figure 8 pattern over the course of a year at the same time of the day. This “Figure 8” is called an analemma.
  • Composite photograph
  • Images taken always at same time of day. (8:30 am. EST)
  • Pictures taken at 10 day intervals over an entire year.







Path of sunrise for a particular date.

Dennis di Cicco


Seasons- Geocentric View

The apparent path of the Sun on the celestial sphere, showing how the seasons arise.

Figure from Astronomy Today, 3rd ed., Chaisson & McMillan, Prentice-Hall, ©1999

right ascension and declination
Right Ascension and Declination
  • Right Ascension (RA): Analogous to longitude, but on the celestial sphere.
    • It is the east-west angle between the vernal equinox and a location on the celestial sphere.
  • Declination (dec): Analogous to latitude, but on the celestial sphere.
    • It is the north-south angle between the celestial equator and a location on the celestial sphere.

Units of R.A.

360o = 24h

 15o/h

the moon our constant companion
The Moon, Our Constant Companion
  • As the Moon moves through the sky, both its appearance and the time at which it rises and sets change with the cycle of Lunar Phases.
lunar phases
Lunar Phases

Note: Look at top view to understand rise and set times.


We always see the same face of the Moon.

  • The Moon rotates as it orbits the Earth.
  • The rotation rate is synchronized with the orbital period.
  • This is called a “Synchronous Orbit”.
  • In order for an Eclipse to occur:
  • The nodes of the Moon’s orbit must be nearly aligned with the Earth and the Sun.
  • The phase of the Moon must be either new or full.

The Ecliptic Plane is represented by the surface of the pond.

Lunar Eclipse

Solar Eclipse

Solar Eclipse

Lunar Eclipse

Nodes occur at points where the orbit crosses the surface of the pond

geometry of a lunar eclipse
Geometry of a Lunar Eclipse

(Sunlight is partially blocked)

(Sunlight is fully blocked)


Multiple Exposure Photograph of Progression of a Solar Eclipse

Totality- lasts only a few minutes.

The Solar Corona becomes visible.

how often do eclipses occur
How often do Eclipses Occur?
  • There are two periods each year when the nodes of the Moon’s orbit are nearly aligned with the Sun.
  • These are called Eclipse Seasons.
  • The combined effect of the changing dates of eclipse seasons and the 29.5 day lunar cycle, makes eclipses recur about every 18 years.
  • This 18 year cycle is called the Saros Cycle.

Solar Eclipses from 2001 to 2022

Colors represent successive Saros Cycles.

the ancient mystery of the planets retrograde motion
The Ancient Mystery of the Planets Retrograde Motion
  • Apparent Retrograde Motion
  • A period during which a planet appears to move westward relative to the stars is called a period of Apparent Retrograde Motion.
  • Prograde (Direct) Motion
  • The usual eastward movement of the planet is called Prograde or Direct Motion.
retrograde motion for jupiter
Retrograde Motion for Jupiter

Retrograde Motion

Prograde or Direct Motion



what causes retrograde motion
What Causes Retrograde Motion?
  • It is a result of Stellar Parallax.
    • Parallax is the apparent shift of the position of a nearby object with respect to a more distant background object.
    • Do pen/pencil against background wall demo.
  • That is, an apparent shift of the planet’s position relative to the background stars.
Explanation of apparent retrograde motion using the principle of parallax.
  • Follow the lines of sight from Earth to Mars in numerical order.
stellar parallax
Stellar Parallax

More distant stars are used as the background for measuring parallax.