Constellations

Patterns in the Sky

The Constellation Orion

• Constellations: a group of stars that seem to form recognizable patterns in the sky

• Light-year: the distance that light travels in one year, about 9.5 x 10 to the exponent 12

• 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.

– Apparent magnitude = the brightness of a star as seen from Earth

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

• 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 or the North Star) = your 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

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

Lunar Eclipse = the phenomenon in which the shadow of the Moon falls on the Earth′s

surface.

(Sunlight is partially blocked)

(Sunlight is fully blocked)

Three Types of Lunar Eclipses

Solar Eclipse = the phenomenon in which the shadow of the Moon falls on Earth′s surface.

Multiple Exposure Photograph of Progression of a Solar Eclipse

Totality- lasts only a few minutes. The Solar Corona becomes visible.

Tides

• The Moon′s motion is responsible for the tides.

• They are caused by the force of gravity; the amount or gravitational force is the force of attraction between all masses in the universe.