How do we know when an eclipse is going to happen? How do we know in advance where it will be visible?
Solar eclipses happen when our view of the sun is blocked by the moon. When the moon lines up between the sun and Earth, the moon will cast a shadow onto Earth. This is what we on the ground observe as a solar eclipse.
We know when they’ll happen because over centuries astronomers have measured very precisely the motions of the Earth, moon and sun, including their orbital shapes, how the orbits precess and other parameters. With those data about the moon – and similar information about the Earth’s orbit around the sun – we can make mathematical models of their movements in relation to each other. Using those equations, we can calculate tables of data that can predict what we will see on Earth, depending on location, during an eclipse as well as when they will happen and how long they last. (The next major solar eclipses over the U.S. will be in 2023 and 2024.)
The path of the eclipse on August 21.
How often do eclipses happen?
A solar eclipse happens, on average, a couple times a year. The moon passes between the Earth and sun every 29 days, a time we call the “new moon” – when the moon is not visible in Earth’s nighttime sky. However, the moon’s orbit and the sun’s path in our sky don’t match up exactly, so at most of those new moon events, the moon appears above or below the sun.
The blue line shows the ecliptic, the path the sun appears to take in our sky as viewed from Earth. The white line shows the moon’s orbit. For eclipses to happen, both the sun and the moon need to be within the area marked with yellow brackets.
John French, Abrams Planetarium, CC BY-ND
Twice a year, though, there is a period where the moon and the sun line up with Earth – astronomers call this an eclipse season. It lasts about 34 days, long enough for the moon to complete a full orbit (and then some) of the Earth. During each eclipse season, there are at least two eclipses visible from some parts of the Earth. At the full moon, there will be a lunar eclipse, when the moon passes directly behind the Earth, resulting in a darker, reddish-colored moon. And at the new moon, there will be a solar eclipse, when the sun is blocked by the moon.
Can we learn anything from eclipse events, or are they really just oddities that happen in nature?
We can definitely learn things from eclipses. The outermost layer of the sun, known as the corona, is difficult to study because it’s less bright than the rest of the sun – so we have trouble seeing it amid the rest of the sun’s brightness.
During an eclipse the sun’s corona becomes visible to observers on Earth.
When the moon blocks the sun, we can see the corona, the famous visual of the halo of light around the dark disk of the moon. Currently astronomers study this by creating an artificial eclipse with a mask built into special instruments on telescopes called coronagraphs. This is great, but doesn’t allow the best pictures. Eclipses give scientists opportunities to get more data to study the corona in depth.
We can also learn about Earth itself. In an area affected by an eclipse, the darkening of the sun leads to a sudden drop in temperature. NASA-funded studies during this eclipse will look at the effects from the eclipse on our atmosphere as well as what happens on land. Previous studies observed animal behavior during an eclipse in 2001 and noted some animals went through their night routines as the sun disappeared while others became nervous.
And we can learn about the whole universe. Less than 100 years ago, an eclipse proved a prediction Albert Einstein had made about gravity. That success helped make him a household name. In his general theory of relativity, Einstein had predicted that gravity could bend the path of light. The effect he predicted was very slight, so it would best be viewed as the light passed a very large celestial body as part of its travels across a very long distance of space.
Sir Arthur Eddington, an astronomer who helped further the study of general relativity and whose work is a major piece of our modern understanding of stars and black holes, used the darkness provided by a solar eclipse to look at the position of the stars’ light during the day, when it passed the sun. He then compared those positions to their known positions at night. He saw that the gravity of the sun had bent the path – exactly as, and in the precise amount that, Einstein had predicted.
How weird is it that the moon can basically exactly block out the sun?
It is very unusual that the moon and the sun just happen to be at the right distances and sizes to appear to have the same size in our sky. This allows the moon to perfectly block the sun’s disk, while also showing us the corona. Venus and Mercury, for instance, can also pass in front of the sun from our perspective. However, they appear as small specks moving across the sun.
Venus appears as a small dot in the upper left as it passes between the sun and Earth in 2012.
What would someone standing on the moon see happen on Earth? Would Earth get dark?
If you were on the moon, you would be able to see the effects of the solar eclipse on Earth only if you were standing on the moon’s night side, the side facing the Earth. You would see a round shadow cast onto the Earth. This particular eclipse will first hit the Pacific Ocean, then move into Oregon, cross the U.S. to South Carolina and end in the Atlantic Ocean. This path the shadow takes is called the path of totality.
Shannon Schmoll, Director, Abrams Planetarium, Department of Physics and Astronomy, Michigan State University
This article was originally published on The Conversation. Read the original article.
I’m not really an eclipse expert, even though I can’t wait for August 21. I’m actually a meteorologist, and a fairly specialized one at that. Six months ago, I didn’t know the difference between an umbra and penumbra. What I did know is that the sun provides energy for everything that happens on our planet, and that the daily cycle of sun rising and setting is a key component of what happens in the atmosphere, and how air circulates locally and globally.
So why is someone who worries about subsecond- and submeter-scale winds interested in this astronomical-scale event? Because any change in incoming sun – such as the complete blackout during a total solar eclipse – will affect the energy received by the land, and in turn the energy transferred back to the atmosphere. And because the total eclipse period is short, those changes will be small. It’s both an exciting event and an interesting challenge: a scientist’s dream.
The mini-night caused by the moon blacking out the sun during the day is an opportunity to investigate many scientific questions.
Coupled with advances in observational techniques, every eclipse offers a new chance to prove meteorological theories. This one even more so because coordination across the entire length of the continental United States almost guarantees that someone will have ideal observing conditions. We’re prepping our weather balloons and weather stations to take advantage of that opportunity – to see exactly what a short blackout does to atmospheric motion.
From how pollutants are formed and transported, to how plants exchange carbon using photosynthesis, to what direction the wind blows, daytime processes are different from nighttime processes. Without energy input from the sun, the lower atmosphere slowly flips itself at night.
During the day, it’s warm near the ground and cooler up above; at night it’s just the opposite. This “stable” (warmer over cooler) air inhibits vertical motion of the air and anything suspended in it. So pollutants can stay closer to the ground, clouds form differently, air flows faster down valleys and at the coasts wind blows offshore instead of on.
While those generalities are known, the nuances and timings aren’t fully understood, and thus they are not completely predictable. That’s my sphere of science – turbulence. I’m interested in the atmospheric changes in short times and small spaces that can eventually influence the larger “weather” most people are familiar with.
The total solar eclipse is a mini-night experience, so we will use it as a natural experiment. Is a brief period without solar radiation enough to cause detectable changes in turbulence and stability, or is it the slower interactions of land and atmosphere over a whole night that are required? We’ll take what we find and use it to think about normal non-eclipse conditions.
The troposphere is the part of the atmosphere closest to Earth’s surface and includes the air we breathe.
NASA ESPO/INTEX-NA Educational Outreach, CC BY
By launching a series of weather balloons before, during and after the eclipse we will see the evolution of winds and temperatures above the Earth’s surface over time. The instrument packages attached to the balloons take measurements from about 100 meters above the surface up through the lower atmosphere, troposphere and lower stratosphere, eventually reaching nearly 20 kilometers. Scientists are coordinating all across the eclipse’s path, and will conduct this same experiment at several sites across the country.
At our site in South Carolina, we are focusing on the question of whether a total eclipse can generate internal atmospheric gravity waves: parcels of air moving together as chunks trying to regain an equilibrium in temperature and density. (These are different from the gravitational waves that result when black holes collide.) Sometimes gravity waves are visible in clouds. During previous eclipses there has been promising evidence of gravity wave activity, but not enough data from enough locations to fully understand them.
The pattern of atmospheric gravity waves is visible in this satellite image of double, overlapping arcs of clouds over the Indian Ocean.
Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC, CC BY
The vertical profiles of temperature, relative humidity, wind speed and wind direction we collect will be used to answer a number of other scientific questions as well. First, we’ll add to the sparse database of eclipse-induced temperature changes and provide quantitative measures of how strong the temperature change is and how long the lag between the total blackness at solar minimum and the temperature minimum is.
We will also be able to see if the cooling when the sun disappears and sudden rewarming when it returns propagates vertically and, if so, how far above the Earth’s surface it goes. In terms of wind, questions to be answered center around changes in wind speed and turbulence intensity. We believe we will see a reduction of both, which provides further explanation for the eerie “eclipse wind” so often cited by human observers.
This more comprehensive examination of the troposphere and stratosphere in time and space will help inform our modeling and prediction of regional weather and climate.
But what if the changes are smaller? A helium-filled balloon leaves the ground quickly – ideally at five meters per second – and the first reliable measurement is almost 100 meters above the ground. A lot can happen in 100 meters.
Postdoc Alexandria McCombs and graduate students Mayra Roman-Rivera and Peter Tereskiewicz work on installing meteorological instruments in preparation for the eclipse experiment.
Ian Giammanco, Insurance Institute for Business & Home Safety,DisasterSafety.org, CC BY-ND
To fill in that gap, at our site in South Carolina, we are adding other measurements. We’ve erected a small tower with fine thermocouples every half-meter from the ground up. These thin wires can detect temperature changes over 0.1-second time periods and will help us see if the darkness causes a very shallow layer of cooler air to start to grow under the typical daytime warmth.
The tower will also house two sonic anemometers – sensors that use disruption in a sound pulse to measure the wind speed in three dimensions at very fast rates – to see if a wind shear develops near ground level.
An infrared gas analyzer will record carbon fluxes throughout the eclipse period to see if there is any detectable change in plant respiration. Remember, they “breathe” in carbon dioxide. Some animals interpret an eclipse as night – do the plants?
The USC backscatter lidar at a recent field deployment in New Zealand.
April Hiscox, CC BY-ND
Finally, we’ll also deploy a lidar system. That’s like a radar, but with a laser that will point upward. This is to see if there are any changes in the depth of the boundary layer – a transition point between where the atmosphere is affected by the Earth’s surface to the free troposphere above.
And we’re going to do all of this in just two minutes and 36 seconds. A tiny window for a big impact.
A practice weather balloon soars above the USC campus.
Patrick Remson, CC BY-ND
A total solar eclipse is often referred to as a meteorological playground, and that is just how it feels. We’re taking out all our scientific toys to see what we can find. Eclipse events are relatively rare; meteorologists like me take what we know about the interactions between land and air to think logically about what will happen during an eclipse. But until we see it, put an equation on it and predict the next one, it still falls into the realm of theory, not reliably predictable weather.
I feel like a kid again – the eclipse has forced me to think about meteorology in a new and different way – just like looking at the world while hanging upside down from monkey bars.
April Hiscox, Associate Professor of Geography, University of South Carolina
This article was originally published on The Conversation. Read the original article.
