When the Moon's shadow glides across the U.S. on August 21st, you'll have have a chance to hear the eclipse as it happens.

Solar eclipses are more than remarkable visual astronomical phenomena; they’re pretty interesting from a radio viewpoint too. Should overcast skies prevail over your location on eclipse day, you can still make some interesting observations using an AM radio.

Dramatic changes can take place in radio reception when day changes into night and vice versa. Perhaps you’ve had the experience of driving in your car at night, listening to some program on the AM dial, when the announcer will identify the station as WBBM in Chicago. This might seem odd if you are listening from Albany, New York, more than 700 miles (1,100 km) from the Windy City. Yet, cases like this happen every night.

A total solar eclipse produces a broad, round area of darkness and greatly reduced sunlight that travels across Earth’s surface in a relatively narrow path during the daytime. Its effect on sunlight’s local intensity is remarkably similar to what happens at sunrise and sunset. Distant radio stations along and near to the path of totality might briefly experience enhanced propagation, thus making long-distance reception possible during a solar eclipse unlike any other time.

We can thank Earth’s ionosphere for natural long-distance radio reception at night. The ionosphere is composed of a set of tenuous, electrically conductive layers that consist of both neutral and charged particles, extending from altitudes of approximately 30 miles (50 km) to more than 250 miles (400 km). The ions present in the ionosphere interact with radio waves in two ways. They can either absorb the waves, thus reducing their intensity and reducing signal strength, or they can refract the waves, changing their direction; conceptually this is akin to a radio-wave "mirror".

The ionosphere’s main refraction layer in the ionosphere, called the F2 layer, is about 180 miles (290 km) above Earth’s surface and is present both day and night. But at a height of some 30 to 60 miles is the D layer, whose impact on radio propagation, especially at lower frequencies, is essentially negative: it absorbs energy from radio signals passing through it to the F2 layer and weakens them.

But as sunset approaches the D layer rapidly loses ionization and essentially vanishes; radio waves are therefore absorbed (depleted) only during the day. So at night, radio waves easily reach the F2 layer, where they are refracted back toward the ground. Likewise, on the way back down, the D layer’s usual obstructions are gone, so the waves reach the ground in a well-preserved state — often many hundreds of miles from the transmitting station.

Amplitude modulation (“AM”) is the oldest system of commercial broadcast transmission. The pioneer AM broadcast service started operation on the low frequencies it still uses, 540 to 1600 kilohertz (kHz). In 1993, the bandpass expanded to include frequencies up to 1700 kHz. AM broadcast stations use powers of 250 watts to 50 kilowatts (50,000 watts) — the maximum power permitted in the U.S.

Listening to distant radio signals is a most interesting hobby and is referred to by amateur radio enthusiasts as “DX’ing.” As already pointed out, radio signals in the commercial 540–1700 kHz AM radio band can be heard for hundreds — sometimes even thousands — of miles under the cover of darkness. This is especially true of the so-called “clear channel” (Class A) radio stations. “Clear channels” are frequencies set apart by international agreement for use primarily by high-powered stations designed to cover wide areas with line-of sight “groundwave” service and, at night, “skywave” service, particularly for remote rural areas.

Scenarios for August 21st’s Eclipse

When the Moon’s shadow sweeps across the U.S. on August 21st, some of the best receptions should occur for people within about 400 to 800 miles (600 to 1,300 km) of the narrow totality path of AM stations that are likewise near or within that path. As maximum darkness approaches a station’s transmitter, the D layer should weaken and the F2 layer more able to refract the signal and support long-distance reception.

Listeners within a few hundred miles of such a location might hear a distant station begin to fade in. The broadcast will build in strength until the area of maximum darkness passes over the station, at which time signal strength should be at its peak. Then, as the area of maximum darkness moves away from the station, its signal will begin to fade away.

According to Nathaniel Frissell (Center for Solar-Terrestrial Research, New Jersey Institute of Technology), similar radio studies done at previous solar eclipses suggest that it’s more important for the shadow to pass close to the transmitter, rather than in the midpoint of the propagation path from the transmitter to the listener. “I suspect,” he notes, “that this process is less symmetric than we think it should be. Maybe it is more important to have the D-region hole near the transmitter so that less signal gets absorbed right away.”

As an example, on March 7, 1970, a total solar eclipse swept northeastward from northern Florida, up the Atlantic coast, to just off of Cape Cod. An amateur astronomer stationed at Greenville, North Carolina, was able to hear WABC in New York (broadcasting at 770 kHz) for about 20 minutes, centered on the time that 96% of the Sun was eclipsed as seen from the Big Apple.

During the total solar eclipse of August 11, 1999, British radio enthusiasts undertook a nationwide program for monitoring enhanced propagation over a wide range of radio frequencies, including shortwave transmissions. More than 1,700 people participated, and the project found that the eclipse definitely had an effect.

Here’s What to Do

Listed below, are 13 clear channel AM radio stations that lie either within the August’s eclipse path (bold type) or close enough to have at least 95% of the Sun’s disk obscured by the passing Moon. Four of these are Class B stations, which direct their signal diametrically away from another station at night which occupies the same frequency, so that it will not interfere. Case in point: KPNW does this to protect KMOX.

50-kw "Clear Channel" AM Radio Stations In/Near Path of August 2017's Total Solar Eclipse Frequency (kHz) Call Sign Location Mid-Eclipse (UT) Mid-Eclipse (local) 650 WSM Nashville, TN 18:28 1:28 p.m. CDT 670 KBOI (B) Boise, ID 17:27 11:27 a.m. MDT 750 WSB Atlanta, GA 18:36 2:36 p.m. EDT 840 WHAS Louisville, KY 18:27 2:27 p.m. EDT 880 KRVN (B)

Lexington, NE 17:57 12:57 p.m. CDT 1030 KTWO (B)

Casper, WY 17:43 11:43 a.m. MDT 1040 WHO Des Moines, IA 18:08 1:08 p.m. CDT 1110 KFAB Omaha, NE 18:04 1:04 p.m. CDT 1110 WBT Charlotte, NC 18:41 2:41 p.m. EDT 1120 KPNW (B) Eugene, OR 17:17 10:17 a.m. PDT 1120 KMOX St. Louis, MO 18:18 1:18 p.m. CDT 1190 KEX Portland, OR 17:19 10:19 a.m. PDT 1510 WLAC Nashville, TN 18:28 1:28 p.m. CDT

Generally speaking, if you are listening for a particular station that is within 800 miles (1,300 km) away, you might be able to hear it near the time that maximum eclipse is occurring over that station’s transmitter. The map below shows the distribution of the stations in the table.

The equipment needed couldn’t be simpler. Any AM receiver can be pressed into service. If the calibration of your radio isn’t too accurate, try tuning in a particular distant station some night before the eclipse(ideally, August 20th). And sweep through the AM band to determine what reception conditions are like. If your receiver has multiple memory channels with a scanning feature, you’re in luck; such a receiver is ideal for checking a lot of frequencies in a hurry.

I invite you to tune around the AM dial during the eclipse. Send your results — the station you heard and the time that you heard it, along with your location — to me at skywayinc@aol.com. I’ll compile all the results and report the results of this experiment at a later date.

(And if you are a Ham radio operator, consider participating in an eclipse-related citizen-science project sponsored by HamSCI and American Radio Relay League.)