Sunrise and sunset bring more than just the sun dipping below the horizon. Scattered sunlight creates three distinct optical phenomena—alpenglow, the belt of Venus, and crepuscular rays—that are visible only when the sun sits just below the horizon, during civil or nautical twilight. Each arises from different light paths through the atmosphere, and knowing when and where to look makes the difference between a glance and a lasting memory.

What is Alpenglow?

Alpenglow appears as a warm, reddish-pink glow that lights up the eastern slopes of mountains just before sunrise or the western slopes just after sunset. The phenomenon occurs when the sun is 3 to 6 degrees below the horizon, during the deeper part of civil twilight or early nautical twilight. Sunlight travels a long path through the atmosphere, where shorter blue wavelengths scatter away, leaving only the longer red and orange wavelengths to illuminate the landscape.

Contrary to popular belief, alpenglow is not direct sunlight hitting the mountain. It is indirect, scattered light. The mountains themselves block the direct sun, but the sky above them glows with that warm hue, and the ground reflects the sky’s colour. True alpenglow fades quickly, often lasting only 5 to 15 minutes. It is most vivid in dry, clear air with minimal haze.

Understanding the Belt of Venus

The belt of Venus is a soft pink or rose-coloured band that arches across the sky opposite the sun, just above the horizon, during civil twilight. Below the belt, the earth’s shadow appears as a dark blue or grey segment. The belt’s colour comes from backscattered sunlight—light that has bounced off the atmosphere and is then reflected back toward the observer.

The belt of Venus is best seen about 15 to 20 minutes after sunset or before sunrise, when the sun is between 2 and 6 degrees below the horizon. It is visible anywhere with an unobstructed eastern or western horizon, but it is particularly striking over flat landscapes or oceans. The phenomenon lasts as long as twilight itself, typically 20 to 30 minutes, and disappears once the sun drops below about 6 degrees, marking the end of civil twilight.

To catch the belt of Venus, look opposite the sunset direction. The earth’s shadow rises from the horizon like a curved dome, with the pink belt sitting directly above it. Use a site like Sunrise.am to check exact twilight times for your location to plan when to step outside.

Crepuscular Rays: Sunbeams that Radiate from the Sun

Crepuscular rays, often called "sunbeams" or "god rays," appear as alternating beams of light and shadow radiating from the sun’s position near the horizon. They occur when sunlight passes through gaps in clouds or through layers of dust and haze, illuminating the atmosphere along the path of the beam. The dark gaps are shadows cast by clouds or terrain.

These rays are most visible during civil twilight, when the sun is just below the horizon, because the contrast between illuminated and shadowed air is greatest. They can also appear at sunrise or sunset with the sun above the horizon, but the effect is strongest when the sun is low. Crepuscular rays can spread across 90 degrees or more of the sky, appearing to converge at the sun’s location due to perspective. They typically last 10 to 30 minutes.

When and Where to See All Three

The three phenomena share a common requirement: the sun must be low, within about 6 degrees of the horizon. The exact timing depends on your latitude and the time of year. At high latitudes, twilight lasts longer—for example, long twilights up north in Reykjavik can stretch for over an hour, giving you extended windows to observe all three effects in succession.

  • Alpenglow: Best at sunrise facing east, or sunset facing west, in mountainous regions with clear air.
  • Belt of Venus: Look opposite the sun, toward the eastern horizon after sunset or the western horizon before sunrise. Ideal over flat terrain.
  • Crepuscular rays: Look toward the sun’s position, but scan the entire sky. Partial cloud cover helps create the beams.

All three are most vivid during the transition between civil and nautical twilight. Civil twilight ends when the sun is 6 degrees below the horizon, and nautical twilight begins at 6 degrees. The three twilights—civil, nautical, and astronomical—each offer different visual conditions, but these phenomena occur in the earlier stages.

The Role of Latitude and Season

Latitude dramatically affects how long each phenomenon lasts. Near the equator, twilight is brief—less than 30 minutes total—so the belt of Venus and crepuscular rays appear and vanish quickly. At mid-latitudes (like London or New York), twilight lasts 30 to 40 minutes. In polar regions, near the solstices, twilight can last for hours or even all night in summer, offering prolonged views.

Season also matters. Near the summer solstice, the sun sets and rises at shallower angles, extending twilight. Near the winter solstice, twilight is shorter at mid-latitudes but longer near the poles. The best times of year for these phenomena in temperate zones are spring and autumn, when the air is often clearer and twilight lengths are moderate but not extreme.

How to Photograph or Observe

Observation requires no equipment—just clear skies and an unobstructed horizon. For photography:

  • Use a tripod for sharp images during low light.
  • Set your camera to manual mode; an aperture around f/8 to f/11 works well.
  • Use a low ISO (100-400) and a shutter speed of 1 to 10 seconds, depending on brightness.
  • For the belt of Venus, use a focal length of 24-50mm to capture the wide arc. For crepuscular rays, a 50-100mm lens helps isolate the beams.

If you are aiming to capture the blue hour too, note that the blue hour occurs during civil twilight when the sun is between 4 and 8 degrees below the horizon—overlapping with the best times for these phenomena.

Wrap-Up

Alpenglow, the belt of Venus, and crepuscular rays are fleeting but reliable gifts of twilight. Check your local twilight times, scan the sky opposite the sun, and pay attention to the horizon. With a bit of patience, you will see them again and again, as long as the sky stays clear and the sun stays low.