Physics 101 Astronomy Day 2 notes Ch. 1 - second half The Zodiac and the seasons Precession Motion of the Moon and eclipses of Moon and Sun Calendars

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

Right now, Aug. 27, 2015, at 2 p.m. the Sun is in Leo, although we can’t see the stars during the day. This can be seen on a sky chart if we set the time to sometime in the day, say 12 noon using &loc=Macomb&alt=215&tz=CSThttp:// &loc=Macomb&alt=215&tz=CST Note: you have to set the date and time to Year 2015 Month 8 Day 27 Hour 14 Minute 10 and click Update. You can also print black on white to save printer ink if you want to take a printed chart outdoors. Compare with the zodiac chart in the previous slide. At midnight, we could go outdoors and see Aquarius high in the sky, opposite the Sun from Earth. (next slide) Try different months to see that the celestial sphere is in different orientations during the year.

Tonight at 11:55 p.m. the sky should look like this Notice that we see Aquarius high in the sky. The sky chart also show us that the “Northern Cross” officially known as Cygnus (“the swan”) is a constellation high in the sky at this time. For “homework” I would like you to go to this site: and use the magnitude charts to see how much light pollution you have at your location. Try this between 8 and 10 p.m. This is a trial run, next week when the Moon is not in the night sky you can do it again and give me your results for credit.

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 and because there are some forces on it from the Moon and planets.

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.

Motion of the Moon and Phases of the Moon The development of the current model of the solar system began with careful measurement of the movement of the Sun and the Moon. To understand this, let’s review the motion and the phases of the Moon, as we currently understand them. When we watch the Moon, it’s shape changes from one night to the next:

From the astronomy picture of the day web site ( link )link

Earth and Moon, separately From Apollo 17 spacecraftFrom Earth

Earth and Moon, in one picture from the Galileo space probe as it traveled by the Earth on its way to Jupiter.

Lunar Phases

Lunar Eclipse (there are many good web resources) (link)(link)

Lunar Eclipse

Solar Eclipse

Solar Eclipse over China

Solar Eclipse over Zambia sequence of photos superimposed to show the movement of the Sun

Solar Eclipse over Antarctica

Solar Eclipse – composite picture

Solar Eclipse – composite picture (cropped)

Solar Eclipse Types

Penumbra and Umbra

Classroom demonstration of umbra and penumbra Use a large shop light, to get a uniform spherical source of light. Then use a round (or even square) object and look at the shadow at various distances from source or screen. (the next screen is black, in order to get a dark room for the demonstration).

Shadow of Moon seen from 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 when the Earth, Moon, and Sun are lined up. This happens when the intersection of the orbital planes passes through the Sun. See below.

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

A partial solar eclipse on Oct. 23, 2014 was visible in the US. See this webpage (a pdf file): Descriptions are on this page: Location data is given here:

Web page at NASA devoted to this eclipse (link)link

Example of a map of the eclipse path, showing the center line and boundaries of the region under the umbra (total) For the total eclipse of 29 March 2006

There is an Atlas showing the path of totality for the total solar eclipse of 2017, from the mreclipse.com website. See You can expect to see extensive coverage in the press, so you will definitely know this is coming up that summer.

There will be a total eclipse of the Moon this fall, on Sept , and you will be able to see the whole eclipse if the weather is clear. See: E2015Sep28/TLE2015Sep28.html E2015Sep28/TLE2015Sep28.html We will schedule a viewing of this, and you can of course see it from other locations.

Transits occur when a planet crosses between the Earth and the Sun, so we see a dark spot cross the Sun. This can only happen with Venus link or Mercury link.link Occultations occur when the Moon blocks out (occults) a planet. LinkLink For a discussion of year dates (BC, AD, BCE, and CE) see

To do: Get the textbook (access cards not needed). Finish Ch. 1. Start Ch. 2 ! Look at the syllabus and the dates. Mark your calendars for exam dates. First exam is exactly 2 weeks away! Welcome back to WIU. Enjoy the weekend. Look at the Moon (not all weekend, though).