Find a Horizon and Savor the Bending of Light

Moon

By: Bob King June 1, 2023 5

Email (required) *

Yes, I would like to receive emails from Sky & Telescope. (You can unsubscribe anytime)

The full Strawberry Moon on June 3rd invites us to experience the refractive power of Earth's atmosphere.

On June 3rd the Strawberry Moon will rise just a few degrees east of strawberry-hued Antares in the constellation Scorpius around sunset. I may be a hardened deep-sky and comet watcher, but every month I look forward to watching the full Moon rise and flood the sky with light. Fortunately, I live near a lake with a great view to the east and a guaranteed front-row seat to this recurring and always moving sight. During a good part of the year, the Moon rises directly from the water, unobstructed by foreground trees or hills — ideal circumstances to see the prismatic prowess of Earth's atmosphere.

Refraction — or light's ability to bend — is ever-present. It paints rainbows, makes stars twinkle, and delivers the precious light of distant galaxies to our eyes when we peer through a refractor. Human vision would be impossible without it.

The principle is simple. When a light beam passes from one medium to another its speed and direction change. Light slows down when entering a denser medium and speeds up when entering a less dense medium. The change in speed alters the beam's direction. When light from a star strikes the Earth's atmosphere at any angle other than 90° (straight overhead), its trajectory bends upward from the horizon in the direction of the zenith.

Light from a star situated directly overhead experiences no refraction. If you see a star at the zenith that's where it is. But gradually lower your gaze toward the horizon, and refraction increasingly comes into play.

It begins subtly. The position of a star at an altitude of 45° shifts a mere 1′ (one arcminute) in the zenith's direction or upward. Even at 10° altitude the difference is only 5.4′ or about one-fifth the apparent diameter of the full Moon. But refraction quickly increases to 9.7′ at 5° , 21.8′ at 1°, 25′ at 0.5°, and 33.7′ (bigger than the full Moon) at the horizon. That's where the fun begins.

Refraction increases with the density of the medium. The amount of air directly overhead is 1 airmass by definition. At 30° above the horizon, it increases to 2 airmasses with minimal effect. But along the horizon we're looking through 40 airmasses, enough air to refract or "lift" the entire lunar disk into view even before it's risen. At that instant, if you could snap your fingers and make the atmosphere disappear, so would the Moon! On an airless Earth, we would have to wait an additional two minutes for every moon- and sunrise. Refraction likewise delays their setting time by the same amount. That's why daylight is actually a few minutes longer than nighttime on the so-called equinoxes.

Differential refraction neatly explains why the Moon looks like a beanbag chair at rising and setting. Atmospheric refraction at the horizon is ~34′ but diminishes to ~25′ just 0.5° above it. The bottom portion of the Moon, being closer to the horizon, gets "pushed" into the less refracted top. The difference of ~9′ from bottom to top is equal to almost a third of its diameter.

Flattening is even more dramatic from orbit. Astronauts on board the International Space Station see the setting Moon and Sun across a longer line of sight than the view from the ground. Correspondingly greater amounts of refraction flatten each body further into red M&M candies.

Atmospheric refraction is not a constant but varies according to temperature, humidity, and barometric pressure. That's why you won't ever see sunrise and sunset times listed to the second. Under the right circumstances, extreme temperatures and pressures refraction can increase refraction by 2° or more. In his book South!: The Story of Shackleton's Last Expedition 1914–917, polar explorer Sir Ernest Shackleton describes the reappearance of the Sun after the date of its predicted last appearance:

"The sun, which had made 'positively his last appearance' seven days earlier, surprised us by lifting more than half its disk above the horizon on May 8 [1915]. A glow on the northern horizon resolved itself into the sun at 11 a.m. that day. A quarter of an hour later the unseasonable visitor disappeared again, only to rise again at 11.40 a.m., set at 1 p.m., rise at 1.10 p.m., and set lingeringly at 1.20 p.m. These curious phenomena were due to refraction, which amounted to 2° 37′ at 1.20 p.m. The temperature was 15° below zero Fahr., and we calculated that the refraction was 2° above normal. In other words, the sun was visible 120 miles further south than the refraction tables gave it any right to be."

I have not specifically looked for changes in the degree of refraction at the horizon from one full Moon-rise to the next, but I'm curious if routine observation and careful timing of successive moonrises (or sunrises) might expose this variability. Even a half-degree's difference should be easily noticeable with the naked eye. Has anyone else ever seen this?

Besides the gross effects of refraction at moonrise and sunrise, layers of air of different temperature and density along an observer's line of sight also get into the refractive act. Each layer bends light to a varying degree which can fray the Moon's edges, create moving ripples across its face, and greatly distort its shape. Mirages may also be present.

At full Moon, which occurs on Saturday, June 3rd at 11:42 p.m. EDT (3:42 UT on June 4th), we also have the chance to witness Earth's shadow and the Belt of Venus aligned with the rising Moon.

Be aware, though, that from many U.S. locations, the Strawberry Moon will rise very close to sunset, lessening its contrast with the surrounding sky. I hope you'll also make a date with the Moon in the nights that follow, when the waning gibbous will rise in a much darker sky. Find a spot with an unobstructed southeastern horizon and wait for the Moon to get bent out of shape.

Anthony Barreiro

June 1, 2023 at 5:52 pm

Thank you Bob. This is fascinating!

In celestial navigation you start by measuring the angle above the horizon of a celestial object (the Sun or Moon during the day; a planet, star, or the Moon during twilight) and note the time, then you use information from the Nautical Almanac to calculate the point on the surface of the Earth where the object was directly overhead at the time of your observation. Further spherical trigonometry allows you to calculate your position relative to that point. Everything depends of the accuracy of the observed height of the object above the horizon, and that angle is always affected by atmospheric refraction. So you have to adjust every observation for atmospheric refraction, usually with a standard table, sometimes with additional corrections for temperature and atmospheric pressure under more extreme conditions. A rule of thumb is to always use objects that are well above 10 degrees above the horizon; below that refraction is too extreme and variable to provide an accurate position.