Physics 101 Astronomy

• Dr. Brian Davies

• Office: 532 Currens Hall, 298-1307

• Office Hours: M Tu W F 11-noon

• E-mail: BM-Davies@wiu.edu

• http://frontpage.wiu.edu/~bmd111/

• See the webpage for notes & syllabus

Physics 101 Astronomy

• Quick review of Chapters 1 and 2, followed by the first exam.

• You will only need to put your name and ID number on the exam

• Pencils are available – ask for one.• Turn in exam sheet and SCANTRON

and sign your name on signature sheet.• Exit through the door in front-right.

What can we see in the visible sky?

• Humans can see about 6000 stars in the night sky (with good vision and a very dark clear night).

• Some of these form patterns called asterisms.

• These have been grouped into constellations (there are 88 of these in the current system).

The Constellation Orion, as seen in the sky and as imagined.

The Constellation Orion is actually three dimensional, but appears to us as a group of points on the “celestial sphere”

The Celestial Sphere appears to rotate around us at night. But you know that it is the Earth that is rotating.

To observers who think the earth is stationary, The celestial sphere appears to be rotating.

The Northern Sky, in a time exposure, shows the apparent motion of the northern part

of the celestial sphere around the Pole star, Polaris.

Right Ascension and Declination are used to indicate positions on the celestial sphere. They correspond to

latitude and longitude on the surface of the Earth.

The celestial sphere is oriented with respect

to the earth, with poles and an equator.

On the celestial sphere we use Declination like we use Latitude on the Earth.

On the celestial sphere we use Right Ascension like we use Longitude on the Earth, but

measured in hours, minutes, and seconds.

To an observer on the ground, directions are defined in this figure.

Seen from far above the North Pole, the Earth appears to be rotating counterclockwise (CCW).

Sun

If the Sun is directly above point A, then it is local noon there, and in 24 hours it will again be noon at that location on the Earth.

The Earth is also in orbit around the Sun,

taking 365.25 days to revolve once around.

This orbital motion is also CCW if viewed from above the north pole.

In 24 hours, which is called the solar day, the Earth must rotate more than 360 degrees!

Solar vs. Sidereal Day

• The solar day is 24 hours long, by definition, but Earth actually rotates through an angle of 360.986o in order to be aligned with the Sun. This is due to the orbital motion of the Earth, which means that the Earth has to rotate an additional 360o/365 or 0.986o per solar day.

Solar vs. Sidereal Day

• The sidereal day is, by definition, the times it takes the Earth to rotate around and come back into alignment with the stars. This is a rotation of exactly 360o and this takes 3.9 minutes less than 24 hours.

• 1 sidereal day = 0.9973 solar days.

The Zodiac is the set of constellations that the Sun appears to go through during the course of one year.

The Ecliptic is the path of the Sun on the celestial sphere, which is tilted with respect to the celestial equator,

due to the tilt of the Earth’s axis with respect to our orbit.

The axis of the Earth is not perpendicular to the plane of the orbit of the Earth around the Sun.

The Earth is tilted by 23.5o.

Sequenceof photosof the Moon shows the Phasesof the Moon

Lunar Phases

Lunar Eclipse

Lunar Eclipse

Solar Eclipse

Solar Eclipse Types

Penumbra and Umbra

Shadow of Moon seenfrom Mir space station

Animation of Moon eclipsing the Sun, as seen from inside the umbra.

Animation of the view from the dark side of the Moon, looking down on the Earth during a solar eclipse.

Eclipse Geometry is favorable close to the equinoxes.

Eclipse Tracks (also see NASA Eclipse page, Mr. Eclipse and Eclipser)