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.

• Asterisms: Recognizable patterns of stars that are not one of the constellations are called Asterisms:– Big Dipper, Little Dipper, Summer Triangle

The Celestial Sphere

• 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 WayThe band of light called the “Milky Way” traces the galactic plane

as it appears from our location in the outskirts of the galaxy.

From: http://skyphoto.com/milky/Default.htm

© Jerry Schad

The Circling Sky

• Earth rotates from west to east.

• The celestial sphere appears to rotate east to west.

Apparent path of a particular star

The Dome of the Sky:Definitions in your local sky

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.

Angular size of moon ~ ½ o

Angular distance between the “pointer stars” ~ 5o

To Polaris

• 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.

Latitude and Longitude

• 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.

Different sets of constellations are visible in northern and southern skies.

Counter-Clockwise Rotation Clockwise Rotation

Northern Hemisphere Southern Hemisphere

The Altitude of the celestial pole (Polaris) = your 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• 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 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.

SouthNorth Composite photograph

•Images taken always at same time of day. (8:30 am. EST)

•Pictures taken at 10 day intervals over an entire year.

Summer

Fall

Spring

Winter

Dennis di Cicco

Path of sunrise for a particular date.

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

Seasons- Geocentric View

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

• 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 Demonstration

The Moon The Earth

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”.

Eclipses

• 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.

Lunar Lunar EclipseEclipse

Solar EclipseSolar Eclipse

Solar EclipseSolar Eclipse

Lunar EclipseLunar Eclipse

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

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

Geometry of a Lunar Eclipse

(Sunlight is partially blocked)

(Sunlight is fully blocked)

Three Types of Lunar Eclipses

Three Types of Solar Eclipses

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?• 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

• 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

East West

Prograde or Direct Motion

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.

Retrograde Motion Demonstration

• Explanation of apparent retrograde motion using the principle of parallax.

• Follow the lines of sight from Earth to Mars in numerical order.

Stellar Parallax

More distant stars are used as the background for measuring

parallax.

End of Section