The Big Dipper: various views and epochs

The Celestial Sphere

• The stars appear to be fixed to the surface of a celestial sphere

• The Earth rotates eastward – the stars appear to move east to west with diurnal motion of the sky

• The celestial sphere contains all of the visible Universe

• Half of the celestial sphere is visible from any location, but different halves for different locations and times of the day

• The local horizon depends on latitude and time of day – stars not visible unless sun is below local horizon

Long time exposure view to the north above new Gemini telescope, Mauna Kea, Hawaii – 19.50 N

Long exposure looking south from Mt. Kilimanjaro, Africa –30 S

Motion of the Stars

North pole 51 ½ N latitude

Equator

Three Views from a Mid-Northern Latitude

Constellations help chart motion of stars

Note circumpolarstars and constellations –always above the horizon

Can you predict the angle that the paths make with the horizon?

Constellations

Use planetarium software or web resources (www.heavens-above.com) to see sky motions

Constellations

Constellations have been associated with great heroes, creatures, gods in all ancient civilizations – modern usage is Greek names translated into Latin.

Useful for keeping track of time, location, seasons – used by navigators for centuries

Sagittarius and Scorpius

Perseus and Andromeda

Ancient sky charts had poorly defined constellation boundaries.

Modern convention has well-defined boundaries –celestial sphere divided into 88 constellations.

from 1837

Most star names come from Arabic

Star Names

Star NamesSince only 24 brightest stars in each constellation can be denoted by Greek letters…

Most stars numbered in catalogs, e.g., Henry Draper catalog

HD 100546, in constellation Musca, south celestial sphere. APOD 2001 May 2.

Swirling spiral-shaped disk ongoing planet formation?

The Yearly Motion of the Sun

Summer solstice – approx. June 21.

Sun appears 23.50 above celestial equator.

Summer in the Northern hemisphere, winter in Southern hemisphere.

Winter solstice – approx. Dec. 21.

Sun appears 23.50 below celestial equator.

Winter in the Northern hemisphere, summer in Southern hemisphere.

The Cause of Seasons

Can this explain the weather on June 21, Dec 21?

Equal incoming energy spread over differing areas.

The Yearly Motion of the Sun

The Sun completes a 3600 motion on the celestial sphere due to Earth-Sun orbit, but this path (the ecliptic) is offset from the celestial equator by 23.50.

What does the daily motion of the Sun look like at different times of year, from different locations on Earth?

The Daily Motion of the SunLike the stars, the Sun undergoes a diurnal motion due to the Earth’s rotation. This is independent of the slow yearly movement across the CS.

The daily paths of the Sun at latitude 400N. Notice the person’s shadows.

The summer (winter) Sun is above the horizon for more than (less than) 12 hours. The summer Sun stands higher in the sky at mid-day. However, note that it remains below the zenith at all times.

On equinoxes, the Sun moves along the celestial equator, from east to west – 12 hours each of daylight and night.

Motion of the Planets

Mercury and Venus are often visible just after sunset in the western sky or just before sunrise in the eastern sky.

Mercury and Venus are the only planets that are always within a fixed angular distance from the Sun. Why?

The other planets may be seen in the eastern sky at sunset and the western sky at sunrise.