Physics 101 Astronomy Dr. Brian Davies Office: 532 Currens Hall, Office Hours: M Tu W F 11-noon See the webpage for notes & syllabuswebpage

“Discovering the Essential Universe", by Neil F. Comins, 4th ed. Ch. 1: Scales of the Universe The Celestial Sphere The Seasons

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 (88 in the current system). Most have old names from mythology; those in the southern hemisphere have Western names.

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

Why constellations? Let’s have a question and answer session, and ask, why do people invent constellations in the night sky? Before indoor lighting, people spent their evenings under the stars, with plenty of time to talk and observe.

Constellations are invented for Story-telling, mythology, ritual Culture and religion Navigation and timekeeping Artistic depictions are known. Mapping the sky for modern astronomy uses the constellations to label areas delimited by boundaries.

Navigation The pole star, Polaris, can be used to determine the direction toward the North pole. Hydra may have been used by Minoan sailors to get East-West directions (2400 BC). Polynesian sailors used celestial navigation.

Constellations can be used to divide up the sky into regions.

The Celestial Sphere The distance to the stars is not evident to our eyes, and they appear to be at the same distance. If we think of this distance as a radius, the stars appear to be on a sphere, with us at the center.

The Constellation Orion is actually three dimensional, but appears to us as a group of points on the “celestial sphere” For more images, see this link, which will be given in the class notes for the day, available from my website. (link)link(link)

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

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.

Macomb, Illinois o N, o W

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.

The observer on the ground can also use angles For altitude (Alt), the angle up from the horizon, and azimuth (Azm), which is a heading (N, E, S, W etc.).

For more detail on Right Ascension and Declination, see this.this

Stars are (almost) fixed on the celestial sphere and the Earth rotates inside this sphere. It only looks like the celestial sphere is rotating from our perspective if we are on the Earth.

Question ? To us, which way does the celestial sphere appear to be rotating? Hint, think about the motion of the moon and the sun during the day or night. Now, if the celestial sphere is actually not moving, and the Earth is rotating, which way does the Earth rotate?

Apparent rotation of the celestial sphere is due to the rotation of the Earth. The Earth is rotating around an axis that goes from pole to pole through a center. Eventually, each day, the Sun sets in the west. If we suppose the Sun is the center of the solar system, it is fixed, so: Each point on the surface of the Earth is going east all the time ( except the poles ).

View of sunset from 100,000 km above the Earth, as it rotates. Get latitude and longitude from a web site: Then put into Earth-Moon viewer: Start view 100,000 km above Macomb, IL at midnight UTC, June 7, 2005 (in local time, 7:00 p.m., June 6, 2005).

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 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 o 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 360 o /365 or o 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 360 o and this takes 3.9 minutes less than 24 hours. 1 sidereal day = solar days.

Why is this relevant ? The difference between the solar day and the sidereal day means that the Sun and the stars appear to be going around the Earth at different rates. The Sun goes around in 24 hours. Stars go around in 23 hours, 56.1 min. So the Sun is not in the same place on the celestial sphere day after day.

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 23.5 o.

Seasons are due to the changing orientation of the Earth and Sun, not because the Earth is orbiting closer or further from the Sun.

Precession of a top We can demonstrate a type of motion called “precession” by recalling the motion of a toy top (a wobbling motion). A bicycle wheel can be used to demonstrate precession. The Earth precesses because it is not a perfect sphere.

Precession of the Earth

Precession of the Earth takes 26,000 years.

The North Celestial Pole moves around a circle on the celestial sphere over long periods of time.