Physics 101 - Astronomy

Study guide and notes for Exam # 1.

Here are some factual statements that you might want to know for the exam. Some of the material is best seen in the figures in the book. You should also look at the notes for each day of the course. The readings are listed at the bottom of this page.

The 88 constellations are based on ancient mythology (in the northern hemisphere), as well as more recent designations (in the southern hemisphere). Present-day astronomers consider constellations as regions of the sky. The stars are actually at very different distances from Earth. The Celestial Sphere is a representation of what we see if we ignore the actual distances of the stars. A long-exposure photo of the Northern Sky shows the apparent rotational motion of the celestial sphere. To observers who think the Earth is stationary, the celestial sphere appears to be rotating.

Right Ascension and Declination are used like the longitude and latitude that we use on maps of Earth. For Earth we use the Longitude and Latitude to indicate location on the spherical object. 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. We use Right Ascension like we use Longitude on the Earth, but measured in angular units called hours, minutes, and seconds.

The apparent motion of the Sun is the most noticeable feature of the sky. We know that this is due to rotation of the Earth on its axis. But to understand the apparent motions of the Sun and planets on the celestial sphere, we have to consider that the Earth is also revolving around the Sun in an orbit.

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

If the Sun is directly above a point on the surface of the Earth, then it is local noon there (solar noon, not the standard time noon), and in 24 hours it will again be local noon at that location on the Earth. This is called the solar day. In 24 hours, the solar day, the Earth must rotate more than 360 degrees!

The solar day is 24 hours long, by definition, but the Earth actually rotates through an angle of 360.986 degrees 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 degrees/365 = 0.986 degrees per solar day.

The sidereal day is, by definition, the time it takes for the Earth to rotate around and come back into alignment with the stars. This is a rotation of exactly 360 degrees and this takes 3.9 minutes less than 24 hours, so 1 sidereal day = 0.9973 solar day.

The Zodiac is the group of 12 constellations on the ecliptic, which is the path of the Sun on the celestial sphere

The axis of the Earth is not perpendicular to the plane of its orbit around the Sun. The Earth is tilted by 23.5 degrees.

Seasons are due to changing orientation of the Earth and Sun, which happens because of the tilt.

The zenith is the point in the sky directly above an observer on the ground. The location of the celestial equator in the sky depends on the observer’s latitude.

The North Celestial Pole and the Celestial Equator are tilted at an angle from the horizon and zenith, respectively, and this angle equals the latitude.

The ecliptic is a circle tilted 23.5 degrees from the celestial equator. The Sun moves along this ecliptic and goes all the way around in one year, thus the Sun appears to be higher in the sky than the celestial equator for 6 months and lower for the other 6 months.

Because the Sun is higher in the sky during the summer, the intensity of sunlight on the ground is greater. This is just due to the fact that the same amount of sunlight hits a smaller area.

Lunar Phases - see your book to review the names of the phases of the Moon. Learn these.

Eclipses - again, see the figures and review the definitions of umbra and penumbra, total and partial eclipse.

Retrograde motion - see the figures in the book or my PowerPoint slides. You should see how retrograde motion can be explained by either the geocentric model or the heliocentric model.

Galileo’s observation that Jupiter has moons provided support to the idea that Jupiter and the Earth were both planets in orbit around the Sun, each with moons of their own.

Venus has a cycle of phases, which could only be seen by a telescope, and because of the similarity to the phases of Earth’s Moon, and some reasoning about the orbits, Galileo deduced that Venus revolves around the Sun in an orbit smaller than Earth’s orbit.

Galileo Galilei used a telescope (which he built in 1609) to observe:
1. The Moon has mountains, valleys, and craters (so it may be like the Earth).
2. Sunspots move around about once per month (showing that the Sun rotates).
3. Jupiter has moons (he saw 4) providing support to Copernicus.
4. Venus has a cycle of phases (like the Moon), implying it revolves around the Sun, as in the model of Copernicus.

Kepler’s First Law: The orbits of the planets are ellipses. The Sun is at one of the focal points of the ellipse.

Kepler’s Second Law: An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal intervals of time.

Kepler's Third Law: (Period of the orbit) squared = (semimajor axis) cubed.

Astronomical Unit: A.U. = 150,000,000 km is the distance between the Sun and Earth.

Newton's First Law: the law of inertia. An object will remain at rest or continue moving at a constant velocity unless it is acted upon by an external net force.

Newton's Second Law: the relation between force, mass, and acceleration. This is described in words in the text, and the equation is the famous equation F = ma that is taught in high school physics classes. The acceleration of a mass is proportional to the total force acting upon it, and inversely proportional to the mass of the object.

Newton's Third Law: forces occur in pairs, called action and reaction. For every force acting upon an object (action), there is a force acting on another object (reaction) which has the same magnitude (size) but points (acts) in the opposite direction.

Gravity is a force between objects that have mass. Gravitational force varies with the distance between the objects. It depends on the product of the two masses, i.e., m1 x m2 and on the inverse of the square of the distance between the masses (assuming they are small compared with the distance) 1/r2
The Sun’s gravity causes planets to move on a path called an orbit. These obey Kepler’s Laws, which can be explained on the basis of Newton's Laws.
The orbits of the planets are ellipses, but it is also possible to have orbits which are parabolas or hyperbolas (known as conic sections).


The first exam was postponed due to cold weather and will be given on Tuesday, Feb. 5.

If you are studying from the free online OpenStax Astronomy book, here are the corresponding sections to study. These are in the order we discussed them in class. If you are just starting to study, just read them in order. If you have already seen the material in the book or in class, you might review some vocabulary first: at the ends of Ch. 2, 3, and 4, see the "Key Terms" to build your vocabulary. You may want to review the "Summary" which comes just after the "Key Terms" in these chapters.

Section 1.1 pp. 11-13
Section 2.1 pp. 31-41
Sections 4.1 - 4.3 pp. 103-117
Section 4.5  pp. 120-124
Section 4.7  pp. 129-135
Sections 1.2 - 1.3 pp. 13-15
Section 2.2 pp. 42-49
Section 2.4 pp. 54-61
Sections 3.1-3.2 pp. 69-80
Sections 3.3-3.4 pp. 81-88

In addition, I recommend quickly reading all of Ch. 1. You do not need to know how to do calculations like in the examples.