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Concept Project

Orbit of the Moon & Tides

  • The Moon revolves counterclockwise around Earth and it takes the Moon 27.3 days to complete a revolution around Earth.
    • "As seen from a north polar orientation, the Moon revolves counterclockwise around the Earth, as the Earth and the other planets do around the Sun. Moving at a velocity of 1.03km/s along a slightly elliptical orbit (e =0.0549), it takes the Moon on average 27.32166 days to revolve around the Earth (sidereal period)."


  • Lunar orbit illustration

lunar orbit illustration

Source:"Earth's companion world." Guide to Stars and Planets, Patrick Moore, Philip's, 2nd edition, 2004. Credo Reference,




lunar phases illustration

Source: Haelle, Tara. Seasons, Tides, and Lunar Phases. Rourke Educational Media, 2016. EBSCOhost,



gravitational effects of sun and moon on tides

Source:tides." Astronomy Encyclopedia, edited by Leif J. Robinson, et al., Philip's, 1st edition, 2002. Credo Reference,


Tides are the consequence of any two objects that exert gravitational pull on one another over a long period of time. Basically, each object gently pulls the other object into an egg-like shape, because the gravitational acceleration on one side of the object is larger than on the other side. On Earth, the most observable evidence of this gravitational effect is the changing tides we witness.

Source: "Tides." The Handy Astronomy Answer Book, Charles Liu, Visible Ink Press, 3rd edition, 2013. Credo Reference,

Tides are caused by the gravitational forces of our Moon and Sun tugging on our planet.

  • How do tides on Earth work?

Two cycles of high and low tides occur each day, roughly thirteen hours apart. High j tides occur both where the water is closest to the Moon, and where it is farthest away. At the points in between, there are low tides.

  • How often do ocean tides occur on Earth?

During about a twenty-six-hour period, each point on Earth's surface moves through a series of two high tides and two low tides—first high, then low, then high again, then low again. The length of the cycle is the sum of Earth's period of rotation, or the length of its day (twenty-four hours), and the Moon's eastward orbital movement around Earth (two hours).

  • Does the Sun also influence tides on Earth?

Yes, the Sun also influences Earth's ocean tides, but only about half as much as does the Moon. Although the Sun is many millions of times more massive than the Moon, it is also about four hundred times farther away from Earth than the Moon is. Tidal effects, like gravitational force in general, are very sensitive to changes in distance.

  • What is a “spring tide”?

When the Moon is in its new phase, or its full phase, the Moon, Earth, and the Sun fall roughly along a straight line in space. As a result, the tidal effects on Earth's oceans are magnified compared to other times in the Earth/Moon/Sun orbit. We call tides that occur at these times the “spring tides,” even though they can happen any time of year and have nothing to do with the seasons. The term actually comes from the German springen, meaning “to jump” or “to rise up.”

  • What is a “neap tide”?

When the Moon is in its first quarter or last quarter phase, the line between Earth and the Moon is at right angles to the line between Earth and the Sun. As a result, the tidal interaction between Earth and these two solar system bodies do not work together at all, and the difference between high tide and low tide is the smallest for that month during these times. We call tides that occur at these times “neap tides.”

  • How does the tidal action of the Moon affect Earth?

The liquid core of Earth—and, minimally, the solid part as well—is also pulled ever so slightly backward and forward by the Moon's tidal action. Its motion is tiny (much, much less than ocean tides) but over billions of years that kind of tidal activity is like squeezing a rubber ball in your hand over and over: the core heats up. That heat eventually diffuses through the planet, affecting processes like volcanism and plate tectonics.

  • How has Earth's tidal action influenced the Moon?

Earth's tidal action on the Moon actually causes the Moon's spin to slow down. The Moon used to spin on its axis, just like Earth does today, but tidal forces have drained away a large amount of its spin—known in physical terms as “angular momentum.” Today, the Moon always has the same side facing Earth.

  • What will happen far in the future to the Earth-Moon system because of their mutual tidal action?

If Earth and the Moon were to continue orbiting around one another undisturbed for an indefinite period of time, their mutual tidal action would continue to dissipate their angular momentum. Eventually, Earth will be tidally locked to the Moon, so the same face of Earth will always face the same face of the Moon. Even now, the Moon's tidal action is slowing down the rate of Earth's spin; a million years from now, a day on Earth will be about sixteen seconds longer than it is today."

Source: "Tides." The Handy Astronomy Answer Book, Charles Liu, Visible Ink Press, 3rd edition, 2013. Credo Reference,



  • Illustration of tides and the sun and moon's effects on tides

the tides

Source: "The Earth–Moon System." Atlas of the Universe, Patrick Moore, Philip's, 6th edition, 2003. Credo Reference,


On the side of the Earth closest to the Moon, the gravitational force of the moon is strongest, causing the surface of the ocean to bulge toward the Moon. On the opposite side, the force is weakest, resulting in a bulge away from the Moon caused by the Earth's rotation. These forces and the response of the ocean would result in twice daily, or semi-diurnal, tides perfectly aligned with the Moon were other factors not involved.

Since the Earth is spinning on its own axis, the bulge of the tides is displaced and appears slightly ahead of the Moon's actual position. This displacement is a result of the frictional forces between the water mass and the Earth's surface, which slows the oceans' response to the gravitational pull of the Moon. The Sun also exerts a gravitational force on the surface of the ocean, although this is weaker, around two-fifths as strong as that of the Moon.

The relative positions of the Sun and Moon result in monthly cycles of tidal amplitude. When both the Sun and Moon are pulling together, the tidal range is greatest, resulting in high spring tides, but when the gravitational forces of the Sun and Moon are at right angles to one another, low neap tides result.

The Moon's orbital plane changes on a cycle of 18.6 years, reaching a maximum angle of 28.5° to the Earth's equator. At such times, the Moon changes its position from 28.5°N to 28.5°S of the equator every lunar month. When the angle between the Moon's orbital plane and the equatorial plane are greatest, the two daily tidal heights are different, with one very high tide and one smaller high tide.

Source: "The lunar cycle." Guide to the Oceans, John Pernetta, Philip's, 1st edition, 2004. Credo Reference,

neap tide

neap tide

Although the Moon is very massive—73.5 billion billion metric tons—it is so far away from Earth (240,000 miles or 384,000 kilometers) that it has very little gravitational pull on objects at or near Earth's surface. It produces about 1/300,000th the gravitational acceleration that Earth produces at its own surface—far too weak to be felt by any person.

Does the Moon's gravity affect Earth at all?

Definitely! Although the Moon's gravitational pull at any one place on Earth is very weak, the combined effect of the Moon over a large area or volume on Earth can be very noticeable. The Moon's effect is most easily seen in the ocean tides."

Scalable Vector Artwork / Scalable Vector Graphics


All of the files below are from Scientific and Medical Art Database. More images are available. 

Other Formats .eps or PDF

From: Public Domain Vectors  Click on images to download .eps or PDF file.








Nervous System


Nucleus Medical Media. "The Nervous System." Smart Imagebase. 5 Mar 2020 10:32 EST. Nucleus Medical Media. 5 Nov 2020 <>. 




Human skeleton. Multiple images. WkiCommons. .svg format Source: Not authoritative but Creative Commons copyright.


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Cross-Species Heartbeats


  • Humans
    • Source: "Heart." Gale Encyclopedia of Nursing and Allied Health, edited by Jacqueline L. Longe, Gale, 4th edition, 2018. Credo Reference, Accessed 04 Nov. 2020.
      • "Within an average person, the heart beats about 72 beats per minute. Thus, in an average lifespan, the heart beats over 2.5 billion times."
    • Source: "Heart Rate."  Scanlon, Valerie, and Tina Sanders. Essentials of Anatomy and Physiology, F. A. Davis Company, 2014., p. 316. ProQuest Ebook Central, 
      • A healthy adult has a heart rate of 60 to 80 beats per minute. 
  • Miscellaneous
    • "10 Amazing Animal Heart Facts.Veterinary Medicine News,  North Carolina State University, 13 February 2018. 
      • "The cheetah is one of the fastest land animals, but its resting heart beat is about 120 beats per minute, similar to a jogging human. Here’s the difference: While it takes some time for a human heart to reach its limit, usually 220 BPM, the cheetah can go up to 250 BPM in just a few seconds."
      • "The smallest known mammal by mass, the Etruscan shrew weighs in at under 2 grams and has a 25 beats per second heart rate. That’s a 1,500 BPM."
      • "An emperor penguin’s heart rate dips 15 percent from its resting rate when diving and drops even more during long dives (in between dives it jumps rapidly, likely to replenish tissues with oxygen). A manatee heart rate cuts by half while on a long dive and seals decrease their heart rate from 50 to 80 percent while diving. By the way, seals eat squids, which, like octopuses, have three hearts."
      • "There are also numerous animals with no hearts at all, including starfish, sea cucumbers and coralJellyfish can grow quite large, but they also don’t have hearts. Or brains. Or central nervous systems."
    • "Resting Heart Rates." Merck Veterinary Adapted from Detweiler DK and Erickson HH, Regulation of the Heart, in Dukes' Physiology of Domestic Animals, 12th ed., Reece WO, Ed. Copyright 2004 by Cornell University.
      • Species

        bpm (range)





        Chicken (adult)


        Dairy cow








        Guinea Pig
















        Rhesus monkey (anesthetized)




  • Watkins, Thayer. "Animal Longevity and Scale." San Jose State University, 
  • Hummingbirds 
    • Source: "How Fast Does a Hummingbird's Heart Beat?" West, George C., and Carol A. Butler. Do Hummingbirds Hum? : Fascinating Answers to Questions about Hummingbirds, Rutgers University Press, 2010, p. 37. ProQuest Ebook Central, have the largest hearts of all birds. Heart is about 2% of total body weight.
      • Resting rate about 500 beats / minute
      • Average rate about 1,000 beats / minute