• ## 24-hour Time

24-hour Time indicates the time of the day by counting how many hours have passed since midnight. Instead of using AM/PM to indicate morning and afternoon hours, 24-hour time counts from 00:00 to 11:59 for morning hours (AM) and 12:00 to 23:59 for hours after noon (PM). To convert from 24-hour time to 12-hour time, subtract 12 hours from times greater than 12:00 (these times are PM hours). Thus:

00:00 = 12:00 AM (midnight)

01:00 = 1:00 AM

02:00 = 2:00 AM

11:59 = 11:59 AM

12:00 = 12:00 PM (noon)

13:00 = 1:00 PM

14:00 = 2:00 PM

23:59 = 11:59 PM

• ## Astronomical Twilight

Describes the periods of time when the Sun's geometric center is between 12° and 18° below the observer's horizon. To the casual observer the sky appears fully dark. Astronomical observations of point sources (such as stars) can still be made during Astronomical Twilight, but diffuse sources (such as galaxies) are too faint to be seen.
• ## Civil Twilight

Describes the periods of time when the Sun’s center is less than 6° below the horizon. During this period there is enough ambient light that outdoor activities are possible without the use of artificial light. The brightest stars, as well as planets, such as Venus, are also visible during civil twilight.
• ## Daylight

Daylight describes the period between sunrise and sunset each day. As described in the Science section of this site, it varies greatly throughout the year depending on your latitude.
• ## First Light and Nightfall

First light and nightfall, occur at the transition between astronomical and nautical twilight, when the Sun’s center is 12° below the observer’s horizon. First light marks the moment when the sky first begins to lighten before sunrise, while nightfall marks the moment when light is no longer perceptible in the sky after sunset

• ## Horizon

The horizon is the line separating earth from sky. As such it describes the region of the Earth and sky that is visible to you. Because of the Earth’s curvature, it forms a circle centered under the exact location of your eyes. When you move, your horizon moves with you. Because none of us can occupy the same space at the same time our horizons are always slightly different.

• ## Horizon-Specific

If something is horizon-specific it changes depending on your visible horizon. Because your horizon is dependent on your exact location, all horizon-specific phenomena are also site-specific. The Yearlight Calendar is horizon-specific because all of the events depicted by it, such as sunrise, occur with respect to the horizon.

• ## Local Solar Midnight

Local Solar Midnight, like local solar noon, is the moment when the Sun is directly on the other side of the Earth. It occurs, on average, 12 hours earlier/later than local solar noon.

• ## Local Solar Noon

is the moment in the day when the sun is at its highest point in the sky. Like Sunrise and Sunset, it is a horizon-specific phenomena depending on your longitude. If you are on the central longitude for your time zone (e.g. 0°, ± 15°, ± 30°, ± 60°, ± 75°, ± 90°, ± 105°, ± 120°, ± 135°, ± 150°, ± 165°, ± 180°) your local solar noon will occur at 12:00 (noon)*. If you are East of this central longitude, Local Solar Noon occurs before the clock reads 12:00. If you are West of the central longitude, Local Solar Noon occurs after 12:00.

*Note: slight variations occur due to the Earth's non-circular orbit around the sun.

• ## Moonlight

Moonlight is the period between moonrise and moonset, when the Moon is above your horizon. The intensity of moonlight varies greatly depending on the phase of the Moon. During full moon, when the moonlight is at its brightest, the moon is roughly 500,000 times dimmer than the Sun.
• ## Moonrise and Moonset

Moonrise and moonset are defined as the times when the Moon crosses the horizon and first becomes visible (moonrise) or fully disappears (moonset). Like sunrise and sunset, it is impossible to predict the exact time of the observed moonrise and moonset throughout the year, due to atmospheric refraction and its dependence on local weather. It is standard practice, however, to assume a refraction angle of 0.6°.

• ## Nautical Twilight

Describes the periods of time when the Sun's geometric center is between 6° and 12° below the observer's horizon. During Nautical Twilight sailors are able to take reliable star sightings using a visible horizon for reference. At the beginning of Nautical Dawn and the end of Nautical Dusk an observer may distinguish general outlines of terrestrial features, but the horizon is imperceptible.
• ## Phase of the Moon

The phase of the Moon is a measure of how much of the Moon’s surface is visible from Earth. This depends on the relative positions of the Earth, Sun, and Moon. When the Moon is between the Earth and the Sun it is not possible to see any of the illuminated surface of the Moon. This is called the “new moon”*. When the Moon is 90° from the Sun, half of the illuminated surface of the Moon is visible from Earth. Although this is casually referred to as a “half moon”, technically it is considered a “quarter moon” because only one quarter of the entire Moon’s surface is visible from Earth. When the Earth falls between the Sun and Moon, we experience a “full moon”*. On the calendar, the phase of the Moon is represented by the opacity of the white line between moonrise and moonset.

* In special cases, when the Moon, Earth, and Sun are exactly aligned, the Moon and Earth can cast a shadows on one another causing an eclipse of either the Sun or Moon. Usually, however, the alignment is not exact enough for these eclipses to occur.

• ## Refraction

Refraction is the bending of light as it passes from the vacuum of space into our atmosphere. In general, this occurs anytime light passes from one material to another.

• ## Site-Specific

Site-specific is a term used in the art world to indicate that a piece was made for a specific location (like an outdoor public sculpture). Often the context around the artwork is taken into account while conceiving the work.

• ## Standard Time

Standard Time divides the globe into roughly 24* regions of coordinated time called “time zones”. These zones are made up of 15°-wide bands of longitude. Within each time zone everyone observes the same time at the same moment. As the Earth takes 24 hours to make one full, 360° rotation relative to the Sun, in one hour, the Earth will rotate 15° (360°/24 hrs = 15°/hr). For this reason the time is set one hour later as you move east from one time zone to the next. Conversely, the time is set earlier by one hour as you move west.

*Note: For political reasons, there are closer to 39 time zones who’s borders stray widely from the 15°-wide bands of longitude described here. These discrepancies increase the variations possible within the Yearlight Calendar.

• ## Sunrise and Sunset

Sunrise and sunset are defined as the times when the Sun crosses the horizon and first becomes visible (sunrise) or fully disappears (sunset). Due to the bending of the sunlight as it passes through the Earth’s atmosphere, it is possible to see the Sun before/after it has actually crossed the horizon. As the degree to which the atmosphere bends sunlight is highly dependent on weather, it is impossible to predict the exact time sunrise and sunset will be observed throughout the year. It is standard practice, however, to assume a refraction angle of 0.6°.

• ## Twilight

Twilight is the period of time before sunrise and after sunset in which there is still light in the sky. The period before sunrise is called “dawn” while the period after sunset is referred to as “dusk”. During twilight, sunlight scattering in the atmosphere illuminates the Earth’s surface such that it is neither completely lit nor dark. Twilight is technically defined according to the angle between the observer’s horizon and the center of the Sun. It is divided into three periods: civil twilight, nautical twilight, and astronomical twilight.

Civil Twilight describes the periods of time when the Sun’s center is less than 6° below the horizon. During this period there is enough ambient light that outdoor activities are possible without the use of artificial light. The brightest stars, as well as planets, such as Venus, are also visible during civil twilight.

Nautical Twilight describes the periods of time when the Sun's geometric center is 6° to 12° below the observer's horizon. During nautical twilight sailors are able to take reliable star sightings using a visible horizon for reference. At the beginning of nautical dawn and the end of nautical dusk an observer may distinguish general outlines of terrestrial features, but the horizon is imperceptible.

Astronomical Twilight describes the periods of time when the Sun's geometric center is 12° to 18° below the observer's horizon. To the casual observer the sky appears fully dark. Astronomical observations of point sources (such as stars) can still be made during astronomical twilight, but diffuse sources (such as galaxies) are too faint to be seen.