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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

The Constellation Orion

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  • 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

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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

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The Circling Sky galactic plane as it appears from our location in the outskirts of the galaxy.

  • Earth rotates from west to east.

  • The celestial sphere appears to rotate east to west.

Apparent path of a particular star

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The Dome of the Sky: galactic plane as it appears from our location in the outskirts of the galaxy. Definitions in your local sky

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Angular Sizes galactic plane as it appears from our location in the outskirts of the galaxy.

  • 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.

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To Polaris galactic plane as it appears from our location in the outskirts of the galaxy.

Angular size of moon ~ ½ o

Angular distance between the “pointer stars” ~ 5o

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  • 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.

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Latitude and (Note: This is an angle)Longitude

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  • 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.

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Counter-Clockwise Rotation southern skies.

Clockwise Rotation

Northern Hemisphere

Southern Hemisphere

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

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Altitude and Latitude southern skies.

  • 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!

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Seasonal Changes in the Sky southern skies.

  • The night-time constellations change with the seasons.

  • This is due to the Earth’s orbit around the Sun.

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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

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Seasons- Geocentric View Seasons

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

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Right Ascension and Declination Seasons

  • 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.

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Units of R.A. Seasons

360o = 24h

 15o/h

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The Moon, Our Constant Companion Seasons

  • 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.

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Lunar Phases Demonstration Seasons

The Moon

The Earth

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Lunar Phases Seasons

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

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Eclipses Seasons

  • 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.

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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

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Geometry of a Lunar Eclipse pond.

(Sunlight is partially blocked)

(Sunlight is fully blocked)

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Multiple Exposure Photograph of Progression of a Solar Eclipse

Totality- lasts only a few minutes.

The Solar Corona becomes visible.

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How often do Eclipses Occur? of a Solar Eclipse

  • 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.

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Solar Eclipses from 2001 to 2022 of a Solar Eclipse

Colors represent successive Saros Cycles.

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The Ancient Mystery of the Planets of a Solar EclipseRetrograde 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.

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Retrograde Motion for Jupiter of a Solar Eclipse

Retrograde Motion

Prograde or Direct Motion



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What Causes Retrograde Motion? of a Solar Eclipse

  • 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.

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Stellar Parallax principle of parallax.

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

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End of Section principle of parallax.