Chapter 4 — Earth, Moon, and Sky — Key Terms, Summary, and Further Reading [OpenStax]
Key Terms
- apparent solar time time as measured by the position of the Sun in the sky (the time that would be indicated by a sundial)
- declination the angular distance north or south of the celestial equator
- great circle a circle on the surface of a sphere that is the curve of intersection of the sphere with a plane passing through its center
- International Date Line an arbitrary line on the surface of Earth near longitude 180° across which the date changes by one day
- lunar eclipse an eclipse of the Moon, in which the Moon moves into the shadow of Earth; lunar eclipses can occur only at the time of full moon
- mean solar time time based on the rotation of Earth; mean solar time passes at a constant rate, unlike apparent solar time
- meridian a great circle on the terrestrial or celestial sphere that passes through the poles
- phases of the Moon the different appearance of light and dark on the Moon as seen from Earth during its monthly cycle, from new moon to full moon and back to new moon
- right ascension the coordinate for measuring the east-west positions of celestial bodies; the angle measured eastward along the celestial equator from the vernal equinox to the hour circle passing through a body
- sidereal day Earth’s rotation period as defined by the positions of the stars in the sky; the time between successive passages of the same star through the meridian
- sidereal month the period of the Moon’s revolution about Earth measured with respect to the stars
- solar day Earth’s rotation period as defined by the position of the Sun in the sky; the time between successive passages of the Sun through the meridian
- solar eclipse an eclipse of the Sun by the Moon, caused by the passage of the Moon in front of the Sun; solar eclipses can occur only at the time of the new moon
- solar month the time interval in which the phases repeat — say, from full to full phase
- synchronous rotation when a body (for example, the Moon) rotates at the same rate that it revolves around another body
- tides alternate rising and falling of sea level caused by the difference in the strength of the Moon’s gravitational pull on different parts of Earth
Summary
The terrestrial system of latitude and longitude makes use of the great circles called meridians. Longitude is arbitrarily set to 0° at the Royal Observatory at Greenwich, England. An analogous celestial coordinate system is called right ascension (RA) and declination, with 0° of declination starting at the vernal equinox. These coordinate systems help us locate any object on the celestial sphere. The Foucault pendulum is a way to demonstrate that Earth is turning.
The familiar cycle of the seasons results from the 23.5° tilt of Earth’s axis of rotation. At the summer solstice, the Sun is higher in the sky and its rays strike Earth more directly. The Sun is in the sky for more than half of the day and can heat Earth longer. At the winter solstice, the Sun is low in the sky and its rays come in at more of an angle; in addition, it is up for fewer than 12 hours, so those rays have less time to heat. At the vernal and autumnal equinoxes, the Sun is on the celestial equator and we get about 12 hours of day and night. The seasons are different at different latitudes.
The basic unit of astronomical time is the day—either the solar day (reckoned by the Sun) or the sidereal day (reckoned by the stars). Apparent solar time is based on the position of the Sun in the sky, and mean solar time is based on the average value of a solar day during the year. By international agreement, we define 24 time zones around the world, each with its own standard time. The convention of the International Date Line is necessary to reconcile times on different parts of Earth.
The fundamental problem of the calendar is to reconcile the incommensurable lengths of the day, month, and year. Most modern calendars, beginning with the Roman (Julian) calendar of the first century BCE, neglect the problem of the month and concentrate on achieving the correct number of days in a year by using such conventions as the leap year. Today, most of the world has adopted the Gregorian calendar established in 1582 while finding ways to coexist with the older lunar calendars’ system of months.
4.5 Phases and Motions of the Moon
The Moon’s monthly cycle of phases results from the changing angle of its illumination by the Sun. The full moon is visible in the sky only during the night; other phases are visible during the day as well. Because its period of revolution is the same as its period of rotation, the Moon always keeps the same face toward Earth.
The twice-daily ocean tides are primarily the result of the Moon’s differential force on the material of Earth’s crust and ocean. These tidal forces cause ocean water to flow into two tidal bulges on opposite sides of Earth; each day, Earth rotates through these bulges. Actual ocean tides are complicated by the additional effects of the Sun and by the shape of the coasts and ocean basins.
4.7 Eclipses of the Sun and Moon
The Sun and Moon have nearly the same angular size (about 1/2°). A solar eclipse occurs when the Moon moves between the Sun and Earth, casting its shadow on a part of Earth’s surface. If the eclipse is total, the light from the bright disk of the Sun is completely blocked, and the solar atmosphere (the corona) comes into view. Solar eclipses take place rarely in any one location, but they are among the most spectacular sights in nature. A lunar eclipse takes place when the Moon moves into Earth’s shadow; it is visible (weather permitting) from the entire night hemisphere of Earth.
For Further Exploration
Articles
Bakich, M. “A Timeline of Famous Eclipses.” Astronomy (October 2023): 36. Short thumbnail stories of historical eclipses of note. Also see his “A History of Solar Eclipses” in Astronomy (March 2023): 40.
Bakich, M. “The Next 20 Years of Solar Eclipses.” Astronomy (September 2021): 49. A planning guide for future eclipses.
Coco, M. “Not Just Another Pretty Phase.” Astronomy (July 1994): 76. Moon phases explained.
Gingerich, O. “Notes on the Gregorian Calendar Reform.” Sky & Telescope (December 1982): 530.
Kluepfel, C. “How Accurate Is the Gregorian Calendar?” Sky & Telescope (November 1982): 417.
Harrington, P. “Filters for Observing the Sun.” Astronomy (April 2021): 42. Ways to observe the Sun safely through binoculars and telescopes, for example during partial eclipses.
Krupp, E. “Calendar Worlds.” Sky & Telescope (January 2001): 103. On how the days of the week got their names.
Krupp, E. “Behind the Curve.” Sky & Telescope (September 2002): 68. On the reform of the calendar by Pope Gregory XIII.
MacRobert, A., & Sinnott, R. “Young Moon Hunting.” Sky & Telescope (February 2005): 75. Hints for finding the Moon as soon after its new phase as possible.
Mallama, A. “Understanding Lunar Eclipses,” Sky & Telescope, (November 2022): 34. Clear introduction, with a table of upcoming eclipses.
Pasachoff, J. “Solar Eclipse Science: Still Going Strong.” Sky & Telescope (February 2001): 40. On what we have learned and are still learning from eclipses.
Regas, D. “The Quest for Totality.” Sky & Telescope (July 2012): 36. On eclipse chasing as a hobby.
Schaefer, B. “Lunar Eclipses That Changed the World.” Sky & Telescope (December 1992): 639.
Schaefer, B. “Solar Eclipses That Changed the World.” Sky & Telescope (May 1994): 36.
Shubinski, R. “Defining Time.” Astronomy (December 2021): 54. A brief history of human timekeeping.
Websites
A Walk through Time: The Evolution of Time Measurement through the Ages: https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time. A web-based exhibit from National Institute of Standards and Technology.
American Astronomical Society Eclipse Pages: http://eclipse.aas.org. Compiled for the April 2024 North American total eclipse, these pages have lots of information, images, consumer recommendations, and educational materials good for any future eclipse as well.
Calendars through the Ages: http://www.webexhibits.org/calendars/index.html. Like a good museum exhibit on the Web.
Calendar Zone: http://www.calendarzone.com/. Everything you wanted to ask or know about calendars and timekeeping, with links from around the world.
EclipseWise: Eclipse Information and Predictions: https://www.eclipsewise.com/intro.html. A site explaining both lunar and solar eclipses and giving details of eclipses past and future by an eclipse expert.
Eclipsophile: Weather for Celestial Events: https://eclipsophile.com/. Meteorologist Jay Anderson provides helpful weather predictions for upcoming eclipses, etc.
Future Eclipses: Where, When, and How to See Them: https://www.timeanddate.com/eclipse/. Find all the solar and lunar eclipses coming up, put in the name of your town, and this site shows you the local eclipse circumstances in a nontechnical way.
History of the International Date Line: http://www.staff.science.uu.nl/~gent0113/idl/idl.htm. From R. H. van Gent at Utrecht University in the Netherlands.
How We Divide Time: https://www.rmg.co.uk/stories/topics/why-12-months-year-seven-days-week-or-60-minutes-hour. A page from the Royal Greenwich Museum explains why the week has seven days and the other units we use for time. See also: https://wonderopolis.org/wonder/why-are-there-seven-days-in-a-week.
Lunacy and the Full Moon: http://www.scientificamerican.com/article/lunacy-and-the-full-moon/. This Scientific American article explores whether the Moon’s phase is related to strange behavior.
Moon Phase Calculator: https://stardate.org/nightsky/moon. Keep track of the phases of the Moon with this calendar.
Phases of the Moon Gallery and Information: http://astropixels.com/moon/phases/phasesgallery.html. Photographs, explanations, and calendars for the Moon, presented by a NASA astronomer.
Tides Explained: From NASA: https://moon.nasa.gov/moon-in-motion/earth-and-tides/tides/ and from NOAA: https://scijinks.gov/tides/ (both use animations); from “How Stuff Works”: https://science.howstuffworks.com/environmental/earth/geophysics/tide-cause.htm.
Time and Date Website: http://www.timeanddate.com/. Comprehensive resource about how we keep time on Earth; has time zone converters and many other historical and mathematical tools.
U.S. Naval Observatory: Astronomical Information Center: https://aa.usno.navy.mil/faq/index. The Observatory tasked by the federal government to keep official time and calendars in the U.S. has this site of information, history, and tables for the public.
Videos
Bill Nye, the Science Guy, Explains the Seasons: https://www.youtube.com/watch?v=KUU7IyfR34o. Produced for younger audiences, but college students can enjoy the bad jokes, too (4:45).
Geography Lesson on the Arrangement and History of Time Zones: https://www.youtube.com/watch?v=-j-SWKtWEcU. The basics clearly explained (3:11).
How to Observe a Lunar Eclipse: https://www.youtube.com/watch?v=JRv0tF0te44. A basic explanation of what causes lunar eclipses, why the Moon is red, and how to view; from the California Academy of Sciences (5:29).
Moon Phases: https://www.youtube.com/watch?v=dyDIogWH9uE (4:54). Sped-up video of the Moon and its phases over the course of a year, with a lot of information; NASA produces a new one each year (for a full user guide, see: https://svs.gsfc.nasa.gov/5187/).
Observing Eclipses of the Sun Safely: https://www.exploratorium.edu/eclipse/how-to-view-eclipse. A series of short videos from the Exploratorium, the science museum that partners with NASA to simulcast eclipses from all over the world.
Shadow of the Moon: https://www.youtube.com/watch?v=XNcfKUJwnjM. This NASA video explains eclipses of the Sun, with discussion and animation, focusing on a 2015 eclipse, and shows what an eclipse looks like from space (1:54).
Strangest Time Zones in the World: https://www.youtube.com/watch?v=uW6QqcmCfm8. A history of time zones and examples of places that keep their own time (8:38).
Understanding Lunar Eclipses: https://www.youtube.com/watch?v=lNi5UFpales. This NASA video explains why there isn’t an eclipse every month, with good animation (1:58).
Why Do We Have Different Seasons: https://www.youtube.com/watch?v=WgHmqv_-UbQ. Brief animated explanation of seasons and how the different amounts of sunlight affect lifeforms on Earth. (3:16).
Collaborative Group Activities
- [A] Have your group brainstorm about other ways (besides the Foucault pendulum) you could prove that it is our Earth that is turning once a day, and not the sky turning around us. (Hint: How does the spinning of Earth affect the oceans and the atmosphere?)
- [B] What would the seasons on Earth be like if Earth’s axis were not tilted? Discuss with your group how many things about life on Earth you think would be different.
- [C] After college and graduate training, members of your U.S. student group are asked to set up a school in New Zealand. Describe some ways your yearly school schedule in the Southern Hemisphere would differ from what students are used to in the Northern Hemisphere.
- [D] During the traditional U.S. Christmas vacation weeks, you are sent to the vicinity of the South Pole on a research expedition (depending on how well you did on your astronomy midterm, either as a research assistant or as a short-order cook!). Have your group discuss how the days and nights will be different there and how these differences might affect you during your stay.
- [E] Discuss with your group all the stories you have heard about the full moon and crazy behavior. Why do members of your group think people associate crazy behavior with the full moon? What other legends besides vampire stories are connected with the phases of the Moon? (Hint: Think Professor Lupin in the Harry Potter stories, for example.)
- [F] Your college town becomes the founding site for a strange new cult that worships the Moon. These true believers gather regularly around sunset and do a dance in which they must extend their arms in the direction of the Moon. Have your group discuss which way their arms will be pointing at sunset when the Moon is new, first quarter, full, and third quarter.
- [G] Changes of the seasons play a large part in our yearly plans and concerns. The seasons have inspired music, stories, poetry, art, and much groaning from students during snowstorms. Search online to come up with some examples of the seasons being celebrated or overcome in fields other than science.
- [H] Use the information in Appendix H and online to figure out when the next eclipse of the Sun or eclipse of the Moon will be visible from where your group is going to college or from where your group members live. What time of day will the eclipse be visible? Will it be a total or partial eclipse? What preparations can you make to have an enjoyable and safe eclipse experience? How do these preparations differ between a solar and lunar eclipse? (The website https://www.timeanddate.com/eclipse/ is a useful place for finding out information about future eclipses.)
- [I] On Mars, a day (often called a sol) is 24 hours and 40 minutes. Since Mars takes longer to go around the Sun, a year is 668.6 sols. Mars has two tiny moons, Phobos and Deimos. Phobos, the inner moon, rises in the west and sets in the east, taking 11 hours from moonrise to the next moonrise. Using your calculators and imaginations, have your group members come up with a calendar for Mars. (After you do your own, and only after, you can search online for the many suggestions that have been made for a martian calendar over the years.)
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[Andrew Fraknoi, David Morrison, Sidney Wolff. Astronomy 2e. OpenStax. Mar 9, 2022. Houston, Texas. Book URL: https://openstax.org/books/astronomy-2e/pages/1-introduction Section URL: https://openstax.org/books/astronomy-2e/pages/4-key-terms, https://openstax.org/books/astronomy-2e/pages/4-summary, https://openstax.org/books/astronomy-2e/pages/4-for-further-exploration, https://openstax.org/books/astronomy-2e/pages/4-collaborative-group-activities]