Glossary

The glossary provides a comprehensive overview of terms associated with the sun, sunrise, and sunset, illuminating both everyday and specialized terminologies. Whether you're a casual reader or an avid astronomer, this glossary serves as a concise reference, simplifying the vast expanse of solar and twilight terminologies into easily digestible definitions.

Terms

Alphabetical list of terms included in this glossary, click on each of them to read the description.

Alpenglow
Alpenglow is a natural phenomenon in which the setting or rising sun illuminates mountain peaks, causing them to glow with a reddish or pinkish hue. This effect is particularly striking and beautiful, often seen in mountainous regions during the twilight hours. ^ Go back to list of terms
Azimuth
Angular measurement of an object's direction along the horizon, typically measured clockwise from the north point. ^ Go back to list of terms
Astronomical Twilight
The period when the Sun is between 12 and 18 degrees below the horizon. During this time, the sky is dark enough for astronomers to observe celestial objects without interference from the Sun's light. ^ Go back to list of terms
Blue Hour
Blue Hour is the period just before sunrise and just after sunset when the sky takes on a deep blue hue, offering ideal lighting for photography and atmospheric views. ^ Go back to list of terms
Circadian Rhythm
Circadian Rhythm is the natural 24-hour cycle of physical, mental, and behavioral changes in organisms, largely influenced by light and impacting sleep and alertness. ^ Go back to list of terms
Civil Twilight
The period when the Sun is just below the horizon and there is enough light for most outdoor activities. It begins in the morning, and ends at sunrise and begins in the evening at sunset and ends when the Sun reaches 6 degrees below the horizon. ^ Go back to list of terms
Day Length
The duration of time the Sun is above the horizon in a given day. It varies throughout the year due to the tilt of the Earth's axis. ^ Go back to list of terms
Golden Hour
The period shortly after sunrise and just before sunset during which daylight is redder and softer compared to when the Sun is higher in the sky. It is considered ideal for photography and film. ^ Go back to list of terms
Horizon
The apparent line that separates the Earth from the sky, as observed from any specific location. ^ Go back to list of terms
Meridian
An imaginary line in the sky that runs from the north to the south pole, passing directly overhead. When the sun crosses this line, it is said to be at its zenith. ^ Go back to list of terms
Nautical Twilight
The period when the Sun is between 6 and 12 degrees below the horizon. During this phase, the horizon is still visible at sea, which is useful for sailors to take readings of stars against the horizon. ^ Go back to list of terms
Penumbra
The outer part of a shadow, where the light source is only partially obscured. It surrounds the darker umbra during events like a solar eclipse. ^ Go back to list of terms
Solar Noon
The time when the Sun is at its highest point in the sky for a specific location. It does not always coincide with 12:00 noon on the clock due to the variation in time zones and the equation of time. ^ Go back to list of terms
Solstice
Either of the two times in the year when the Sun is at its greatest distance from the celestial equator, resulting in the longest day (Summer Solstice) or the shortest day (Winter Solstice). ^ Go back to list of terms
Sun Pillar
A vertical shaft of light appearing to extend above (or below) the Sun, typically observed during sunrise or sunset. This optical phenomenon is caused by the reflection of sunlight by tiny ice crystals in the atmosphere. ^ Go back to list of terms
Sunspot
A temporary, dark spot visible on the Sun's surface or photosphere. It appears darker than its surroundings because it is cooler. Sunspots are associated with magnetic activity. ^ Go back to list of terms
Timezone
A region of the Earth that has the same standard time. The concept was established to aid travel and communication across countries and continents that experience different times of the day due to the Earth's rotation. ^ Go back to list of terms
Twilight
The period of time before sunrise and after sunset when the sky is illuminated, but the Sun is below the horizon. It is divided into three phases: civil twilight, nautical twilight, and astronomical twilight, based on the angle of the Sun below the horizon. ^ Go back to list of terms
Umbra
The central, darkest part of a shadow, where the light source is completely blocked by the obscuring body, such as during a solar eclipse. ^ Go back to list of terms
Zenith
The point in the sky directly above an observer. When the Sun reaches this point, it is said to be at its highest position for the day. ^ Go back to list of terms
Polar Night
The period when the Sun is below the horizon for an entire day, typically occurring in polar regions during the winter months. ^ Go back to list of terms
Midnight Sun
The period when the Sun is above the horizon for an entire day, typically occurring in polar regions during the summer months. ^ Go back to list of terms