Tides - Blog Posts
21 Years of Amazing Earth Imagery
On April 29, 1999, NASA Earth Observatory started delivering science stories and imagery to the public through the Internet. Today, we turn 21! So much has changed in the past two decades...
One of the most notable changes is the way we view our home planet. Check out some of the beautiful imagery of our planet over the past 21 years.
2000: Pine Island Glacier
Most people will never see Pine Island Glacier in person. Located near the base of the Antarctic Peninsula—the “thumb” of the continent—the glacier lies more than 2,600 kilometers (1,600 miles) from the tip of South America. That’s shorter than a cross-country flight from New York to Los Angeles, but there are no runways on the glacier and no infrastructure. Only a handful of scientists have ever set foot on its ice.
This animation shows a wide view of Pine Island Glacier and the long-term retreat of its ice front. Images were acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on our Terra satellite from 2000 to 2019. Notice that there are times when the front appears to stay in the same place or even advance, though the overall trend is toward retreat. Read more.
2002: The Blue Marble
In February 2002, Earth Observatory published this “blue marble” image based on the most detailed collection of true-color imagery of the entire Earth at that time. Using a collection of satellite-based observations, scientists and visualizers stitched together months of observations of the land surface, oceans, sea ice and clouds into a seamless, true-color mosaic of every square kilometer (.386 square mile) of our planet. Most of the information contained in this image came from Moderate Resolution Imaging Spectrometer (MODIS), illustrating the instrument's outstanding capacity to act as an integrated tool for observing a variety of terrestrial, oceanic and atmospheric features of the Earth. Read more.
2009: Tsauchab River Bed
The Tsauchab River is a famous landmark for the people of Namibia and tourists. Yet few people have ever seen the river flowing with water. In December 2009, an astronaut on the International Space Station caught this glimpse of the Tsauchab River bed jutting into the sea of red dunes. It ends in a series of light-colored, silty mud holes on the dry lake floor.
Like several other rivers around the Namib Desert, the Tsauchab brings sediment down from the hinterland toward the coastal lowland. This sediment is then blown from the river beds, and over tens of millions of years it has accumulated as the red dunes of the Namib Sand Sea. Read more.
2012: Manning Island and Foxe Basin, Canada
Although it may look like a microscope’s view of a thin slice of mineral-speckled rock, this image was actually acquired in space by the Earth Observing-1 satellite in July 2012. It shows a small set of islands and a rich mixture of ice in Foxe Basin, the shallow northern reaches of Hudson Bay.
The small and diverse sizes of the ice floes indicate that they were melting. The darkest colors in the image are open water. Snow-free ice appears gray, while snow-covered ice appears white. The small, dark features on many of the floes are likely melt ponds. Read more.
2013: A Lava Lamp Look at the Atlantic
Stretching from tropical Florida to the doorstep of Europe, this river of water carries a lot of heat, salt, and history. The Gulf Stream is an important part of the global ocean conveyor belt that moves water and heat across the North Atlantic from the equator toward the poles. It is one of the strongest currents on Earth, and one of the most studied.
This image shows a small portion of the Gulf Stream as it appears in infrared imagery. Data for this image was acquired on April 9, 2013, by the Thermal Infrared Sensor (TIRS) on the Landsat 8 satellite. TIRS observes in wavelengths of 10.9 micrometers and 12.0 micrometers. The image above is centered at 33.06° North latitude, 73.86° West longitude, about 500 kilometers (300 miles) east of Charleston, South Carolina. Read more.
2016: Curious Ensemble of Wonderful Features
When John Wesley Powell explored the Colorado River in 1869, he made the first thorough survey of one of the last blank spots on the map. The expedition began in May at Green River, Wyoming, and ended three months later at the confluence of the Colorado and Virgin Rivers in present-day Nevada.
About two months into their journey, the nine men of the expedition found themselves in Glen Canyon. As the men traveled along the serpentine river channel, they encountered what Powell later described in Canyons of Colorado as a “curious ensemble of wonderful features.”
From above, the view of Glen Canyon is equally arresting. In 2016, an astronaut aboard the International Space Station took several photographs that were combined to make a long mosaic. The water has an unnatural shade of blue because of sunglint, an optical phenomenon that occurs when sunlight reflects off the surface of water at the same angle that a camera views it. Click here to see the long mosaic.
2019: Lena Delta Shakes Off Water
For most of the year, the Lena River Delta—a vast wetland fanning out from northeast Siberia into the Arctic Ocean—is either frozen over and barren or thawed out and lush. Only briefly will you see it like this.
After seven months encased in snow and ice, the delta emerges for the short Arctic summer. The transition happens fast. The animation above, composed of images from the Moderate Resolution Imaging Spectroradiometer (MODIS) on our Aqua satellite, shows the transformation from June 3-10, 2019. Read more.
2020: Making Waves in the Andaman Sea
When tides, currents and gravity move water masses over seafloor features, they can create wave actions within the ocean. Oceanographers began studying these internal waves from ships in the 1960s, and the modern era of satellites has made it possible to see them on a grand scale. The Operational Land Imager (OLI) on Landsat 8 captured these images of the Andaman Sea on November 29, 2019. The reflection of the Sun on the ocean—sunglint—helps make the internal waves visible.
Internal waves form because the ocean is layered. Deep water tends to be colder, denser and saltier, while shallower water is often warmer, lighter and fresher. The differences in density and salinity cause layers of the ocean to behave like different fluids. When tides, currents, gravity and Earth’s rotation move these different water masses over seafloor formations (such as ridges or canyons), they create waves within the sea. Read more.
These images were taken from NASA Earth Observatory!
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This episode’s been a long time coming because the topic’s come up before. I originally conceived of this podcast as a way for me to learn about space things I’d always taken for granted, and truly, there is nothing closer to home that I’ve just agreed to believe than the statement that the tides are affected by the Moon. What? How? Why? All these questions and some I didn’t even realize I had will be answered in this episode on tidal forces!
Below the cut are my standard glossary, transcript, sources, and music credits. Send me any topic suggestions via Tumblr message (you don’t need an account for it!). You can also tweet at me on Twitter at @HDandtheVoid, or you can ask me to my face if you know me. Subscribe on iTunes to get the new episodes of my maybe now monthly-updated podcast (we’ll see how the weeks unfold), and please please please rate and review it. Go ahead and tell friends if you think they’d like to hear it, too!
(My thoughts on the next episode are Stephen Hawking and his theories, or famous comets. The next episode will go up in September—ideally, September 10th!)
Glossary
barycenter - the common center of mass between two objects that allows them to orbit.
Roche limit - the distance in which a celestial body will disintegrate because of a second celestial body's tidal forces exceeding the first body's gravitational self-attraction, or the force that’s holding it together. Within the Roche limit, orbiting material disperses and forms rings, like how Saturn’s rings are within the Roche zone; outside the limit, material tends to coalesce.
spaghettification - when extreme tidal forces pull an object apart in space.
tidal force - an apparent force (sometimes also called the differential force) that stretches a body towards another, more gravitationally-strong body’s center of mass. This can cause such diverse phenomena as tides, tidal locking, breaking celestial bodies apart to form ring systems within a Roche limit, and in extreme cases, spaghettification. It arises because the gravitational force exerted on one body by another is not constant across its parts: the nearest side is attracted more strongly than the farthest side.
Types of ocean tides:
diurnal tide - a daily tidal cycle with only one high and low tide each lunar day, and a period of a little over 24 hours.
meteorological tide - a tidal change due to weather patterns. Wind, or unusually high or low barometric pressure causes variations between the actual sea level and its predicted height.
mixed tide - a daily tidal cycle with two high and low tides that differ in their peaks. This difference in height between successive high or low tides is called the diurnal inequality. They have a period of 12 hours and 25 minutes.
neap tide - a type of bi-monthly tidal cycle that occurs when the Sun, Earth, and Moon are positioned at a 90-degree angle, so the tidal forces of the Sun are acting against the tidal forces of the Moon. During a neap tide, the difference between high tide and low tide is the least extreme.
semidiurnal tide - a daily tidal cycle with two nearly equal high tides and low tides every lunar day. They have a period of 12 hours and 25 minutes.
spring tide - a type of bi-monthly tidal cycle that occurs when the Sun, Earth, and Moon line up so that the gravitational forces of Sun and Moon are working together to form a large tidal bulge. During a spring tide, the difference between high tide and low tide is at its maximum.
tidal locking - when long-term interaction between two co-orbiting astronomical bodies causes at least one of the bodies to rotate in such a way that one face of the body is always pointed at the body it’s orbiting. This is also called gravitational locking or captured rotation. An example is that the same side of the Moon always faces the Earth, and its synchronous rotation means that it takes just as long to rotate around its own axis as it does to revolve around the Earth.
Script/Transcript
Sources
Tidal Cycles in Tides Explained via beltoforian.de
“a tide is a distortion in the shape of one body induced by the gravitational pull of another nearby object.”
Meteorological effects on tides via the New Zealand Government website
Tides and Water Levels via the National Oceanic and Atmospheric Administration (NOAA)
Tides by R. Nave, my dude, my guy, my friend and yours, of Georgia State University
The Tidal Force by Neil deGrasse Tyson via Hayden Planetarium (Nov 1995)
“A mild increase in distance between two objects can make a large difference in the strength of the tidal force. For example, if the Moon were just twice its current distance from us, then its tidal force on Earth would decrease by a factor of eight. At its current average distance of 240,000 miles from Earth, the Moon manages to create sizable atmospheric, oceanic, and crustal tides by attracting the part of Earth nearest the Moon more strongly than the part of Earth that is farthest. (The Sun is so far away that in spite of its generally strong gravity, its tidal force on Earth amounts to less than half that of the Moon.) The oceans respond most visibly in being stretched toward the direction of the Moon.”
“When Earth's rotation slows down until it exactly matches the orbital period of the Moon, then Earth will no longer be rotating within its oceanic tidal bulge and the Earth-Moon system will have achieved a double tidal lock. In what sounds like an undiscovered wrestling hold, double tidal locks are energetically favorable (like a ball coming to rest at the bottom of a hill), and are thus common in the universe.”
Forget “Earth-Like”—We’ll First Find Aliens on Eyeball Planets via Nautilus (Feb 2015)
High Tide on Io! via NASA (Mar 2012)
Tidal forces and spaghettification via NASA handout
Spaghettification via Cosmic Funnies
Single atoms feel tidal force via Physics World (May 2017)
Robbins, Tom. Still Life with Woodpecker. Bantam Books: New York, 1980.
“Being four times larger than the moon, the earth appeared to dominate. Caught in the earth’s gravitational web, the moon moved around the earth and could never get away. Yet, as any half-awake materialist well knows, that which you hold holds you.”
Sobel, Dava. The Planets. Viking: NY, 2005.
Intro Music: ‘Better Times Will Come’ by No Luck Club off their album Prosperity
Background Music: ‘Sad Business’ by Patients aka Ben Cooper, who primarily releases music as Radical Face but also has at least three other bands or band names he’s working with/has released music as.
Filler Music: ‘It’s Getting Boring by the Sea’ by Blood Red Shoes off their album Box of Secrets
Outro Music: ‘Fields of Russia’ by Mutefish off their album On Draught