For complete coverage of the Eclipse of the Century go to cnn.com/eclipse. Watch live, in virtual reality, as the eclipse moves coast to coast on August 21st.

In less than a month, the sun will disappear — for a short time — across America.

For a brief moment, day will turn to night. Animals big and small will go into their nighttime routines. Stars and planets will be visible, and streetlights will turn on in the middle of the day.

Here are some of the things you should know about the total solar eclipse happening August 21.

Don’t miss it! This is rare, says NASA

“The hair on the back of your neck is going to stand up, and you are going to feel different things as the eclipse reaches totality. It’s been described as peaceful, spiritual, exhilarating, shocking,” said Brian Carlstrom, deputy associate director of the National Park Service Natural Resource Stewardship and Science Directorate.

According to NASA, experiencing a total solar eclipse where you live happens about once in 375 years. So, unless modern medicine advances considerably in the next few years, you might not make it to the next one.

The last time anyone in the United States witnessed a solar eclipse was almost 40 years ago, on February 26, 1979. It’s been even longer — 99 years — since a total solar eclipse crossed the country from the Pacific to the Atlantic. The total eclipse on June 8, 1918, passed from Washington to Florida.

You can set your clock to it, even to the precise second.

Make your plans now. If you are reading this at work and want to ask for the day off, you will soon find that all of your science geek colleagues have already asked off for this random Monday in August. If you can’t manage to convey to your boss that no one else will be doing business and you can’t get the day off, block out your calendar for an outdoor meeting or a long lunch

Even if you live in New York, Los Angeles, Houston, Chicago or Atlanta, you will go slightly dark. In fact, all of North America will be able to see a partial eclipse.

Do you have to be in ‘totality’?

To see “totality,” in which the moon completely blocks the sun, you will need to be inside the narrow swath — about 70 miles wide — of the moon’s shadow. The path will stretch from the Oregon coast to the South Carolina coast, with 12 states in between.

Nearly 12.2 million Americans live in the path of totality, but NASA predicts that millions more will visit it that day. “About 200 million people (a little less than 2⁄3 the nation’s population) live within one day’s drive of the path of this total eclipse,” the agency said.

“This will be like Woodstock 200 times over — but across the whole country,” said Alex Young, solar scientist at NASA’s Goddard Space Flight Center.

The Federal Highway Administration is calling this a “planned special event for which there has been no recent precedent in the United States.”

It expects heavy traffic before and after the eclipse along the path of totality. The agency suggests getting to your chosen spot hours before, if not the day before. The one thing you don’t want to do is come up short of totality.

“This is one of those rare events where being close is not good enough,” said J. Kelly Beatty, senior editor of Sky & Telescope. “A sun that’s 99% covered is vastly different than the one that’s 100% covered. Like I say to people, it’s like being on a first date versus being on your wedding night.”

Most astronomers have the same advice: Get to the path of totality, because you won’t want to miss this.

“I know it’s a Monday and for some parts of the country a school day, and it may be inconvenient or cost more than you want, but it really should be a priority,” said David Baron, author of the book “American Eclipse.” “The general impression is, if you live somewhere with a 90% partial eclipse, that’s good enough. Absolutely not. It’s only during a total solar eclipse that you can take off your eclipse glasses, look up where the sun should be with your naked eye and see a sky you’ve never seen before.”

A fast-moving shadow

During a total solar eclipse, the moon and the sun both appear to be about the same size from the ground. According to NASA, this is a “celestial coincidence,” as the sun is about 400 times wider than the moon and about 400 times farther away.

Then, it is just basic geometry. When the Earth, moon and sun line up just right, the moon blocks the sun’s entire surface, creating the total eclipse.

Photographers:  Please be aware that you could damage your camera:

Whereas lunar eclipses are safe to view with the naked eye, solar eclipses are not. You must take the necessary precautions to keep from harming your eyesight. In fact, you also need to use a “solar filter” to keep from harming your camera’s imaging sensor as well as for correct exposure.

A solar eclipse occurs whenever the moon’s shadow falls on Earth. This can only occur during a new moon, when the moon passes between the sun and Earth. There are two or more solar eclipses a year; which occur when the geometry lines up just right, so that part of the moon’s shadow falls on Earth’s surface and an eclipse of the sun is seen from that region.

Solar Filters

When viewing or photographing the partial phases of a solar eclipse or the maximum phase of an annular eclipse, you must use a solar filter. Even if 99% of the sun is covered by the moon, the remaining 1% crescent is dangerous to view with the naked eye and can cause serious eye damage or blindness.

You can find information on solar filters and where to purchase them from astronomy websites and magazines. Safe solar filters for cameras and telescopes are available as either “Full-Aperture” and “Off-Axis” filters. Both of these filters fit over the objective (front end of the telescope) or camera lens.

Full-aperture solar filters are the preferred filters of choice. This is because the filter completely covers the front of the telescope so the entire mirror or lens is used. No refocusing of the telescope or camera lens will be needed when you remove the filter at the beginning of totality or when it is replaced back on the telescope/camera lens at the end of the total phase.

Starting Exposure

Solar eclipses may be viewed and photographed, provided certain precautions are taken. You can photograph a solar eclipse with any type of camera: DSLR, COOLPIX or Nikon 1. The longer the focal length of the lens, the larger the images of the sun you’ll be able to make. While you can also use film cameras to photograph eclipses, this article specifically discusses digital camera use.

With a DSLR, you can also combine a super telephoto lens with a teleconverter to increase the focal length. You can also increase the relative size of the eclipse image by selecting an FX camera’s “DX Crop Mode”. If you’re photographing the solar eclipse using a COOLPIX compact digital camera, turn the built-in flash to OFF.

How large you want the sun to be in the frame will determine what focal length lens to use. For a DSLR camera with a full frame FX sensor, choose a focal length of 2000mm or less. For a DSLR camera that has a DX sensor, the maximum focal length is about 1300mm; any longer and you won’t be able to get the entire sun in the frame.

However, if you also want to capture the sun’s corona during the phase of totality, then you should choose a focal length that’s shorter still—no more than 1400mm for an FX (full frame sensor) camera, or 900mm for a Nikon DX camera.

Place your camera on a sturdy tripod, and manually focus the camera, setting it to infinity.

If you are using a telescope on an equatorial mount, the electric drive will track the sun keeping it centered in your camera throughout the eclipse.

A solar filter must be used on the lens throughout the partial phases for both photography and safe viewing. These filters typically attenuate the sun’s visible and infrared energy by a factor of 100,000. Almost any ISO can be used because the sun gives off abundant light. The actual filter factor and choice of ISO will play critical roles in determining the correct exposure.

The easiest way to determine exposure is to run a calibration test on the un-eclipsed sun on a clear day prior to the eclipse. Digital cameras are ideal as you can see your results almost instantaneously. Shoot the mid-day sun at a fixed aperture, (choose an aperture between f/8 and f/16) using every shutter speed from 1/4000 second to 1/30 second. Looking at the exposures, choose the best shutter speed/aperture combination and use them to photograph the partial phases of the solar eclipse. Your camera’s histogram function is an excellent way to evaluate the best exposure. The histogram should not be clipped but should lie toward the upper end of brightness values. Because the sun’s brightness stays the same throughout the partial phases, no exposure compensation will be needed. You may also decide to bracket your exposures to ensure that you photograph the solar eclipse with a perfect exposure. If you ran your test on a sunny day and the eclipse occurs on a hazy day, increase the bracket of exposures an additional f/stop.

Photographing the Totality Phase of a Solar Eclipse

Certainly the most spectacular phase of the solar eclipse is totality. For a few brief seconds or minutes, the sun’s pearly white corona, red prominences, and chromosphere are visible.

The great challenge is to obtain a set of photographs that captures these fleeting phenomena. During the total phase, all solar filters must be removed. This is because the sun’s corona has a surface brightness a million times fainter than the sun’s visible disk or photosphere, so photographs of the corona must be made without a filter. Furthermore, it is completely safe to view the totally eclipsed sun directly with the naked eye. No filters are needed, and in fact, they would completely hide the view.

The average brightness of the corona varies inversely with the distance from the sun’s limb. The inner corona is far brighter than the outer corona thus, no single exposure can capture its full dynamic range. The best strategy is to choose one aperture and bracket the exposures over a range of shutter speeds from 1/1000 second to 1 second. You should rehearse the actions of setting up the camera and adjusting exposures because it is common for photographers to become easily distracted when viewing this phase of the solar eclipse, so much so that you forget to make pictures.

Whichever exposures you do choose, bracket by one or two f/stops to ensure you get the best possible image. Use RAW format if your camera has this option because it allows greater flexibility in adjusting the exposure when processing your images after the eclipse.

CNN / Judson Jones

If you happened to be sitting on the moon facing Earth, it would look just like the moon is casting a dark circular shadow — called the umbra — on the Earth. This shadow will move across the United States from west to east, but don’t think about trying to keep up with it.

Unless you are flying a fighter jet, you won’t be able to follow the shadow, which will be traveling at almost 3,000 miles per hour when it enters the US and then slow to nearly 1,500 mph when it traverses South Carolina.

A larger and fainter shadow called the penumbra will surround the inner shadow. This is what most people will experience — the partial eclipse.

Precision timing

The lunar shadow first crosses the West Coast at 9:05 a.m. PDT.

People in Lincoln City, Oregon, will be the first in the continental United States to see the total solar eclipse, beginning at 10:15 a.m. PDT.

A total solar eclipse can sometimes take as long as 7½ minutes. The longest eclipse duration for this event will occur in Carbondale, Illinois, and will clock in at two minutes, 43 seconds, beginning at 1:20 p.m. CDT.

Eventually, all good things must come to an end, and the lunar shadow will depart the East Coast at 4:09 p.m. EDT.

This will be the last total solar eclipse in the United States until April 4, 2024.

It’s not quite as long of a wait as you might have thought, but it won’t stretch the width of the country. Instead, it will move from Mexico to Maine and then traverse New Brunswick and Newfoundland.

For another eclipse similar to this year’s, one that moves from coast to coast, you will have to wait until August 12, 2045.

This is a combined article from CNN and Nikon USA.