AS DESCRIBED BELOW, WHAT DRIVES THE YEARLIGHT PHENOMENA IS YOUR POSTION RELATIVE TO THE EARTH, SUN, AND MOON
IMPORTANCE OF LOCATIONIn all the explanations that follow, your latitude and longitude play an important role because they define the region of the sky visible to you at any given moment. In short, they define your horizon. This has a large effect on the timing of sunrise, sunset, moonrise, and moonset because these phenomena are precisely defined relative to your horizon. As you move, your horizon travels with you, giving rise to the unique nature of each Yearlight Calendar.
SUN, EARTH, AND LOCATIONIn general, the site-specificity of daylight results from the fact that the Earth is roughly spherical with an equator that is tilted relative the Earth's orbit around the Sun. This tilt always points in the same direction throughout the year. In June the North Pole faces the Sun, allowing the Northern Hemisphere to receive more than twelve hours of daylight and the Southern Hemisphere to receive less than twelve hours of daylight each day. For this reason June is a warm summer month in the Northern Hemisphere and a cold winter month in the Southern Hemisphere. In December, the opposite is the case: the North Pole faces away from the Sun, causing winter in the Northern Hemisphere and summer in the Southern Hemisphere.
Latitude EffectsBecause of the Earth's tilt, your latitude determines the overall variation of the days throughout the year. The farther you are away from the Equator, the longer the days become in summer and the shorter they become in winter. As mentioned above, depending on whether you are in the Northern or Southern Hemisphere, the longest days of the year will occur either in June (Northern Hemisphere) or December (Sothern Hemisphere).
On your Yearlight Calendar, your latitude therefore affects the visual gradient across the months. If you are in the Northern Hemisphere, the months at the beginning and end of the year will be the darkest, while the months in the middle of the year will be the brightest. This causes a light vertical band in the middle of your calendar. Conversely, if you are in the Southern Hemisphere your calendar will have a dark band running vertically down the middle. You will see this if you compare the calendars in the Example section of this site.
Your latitude also affects the duration of daylight, twilight, darkness and moonlight for any given day. As you move towards the poles, the Sun's path makes a lower angle relative to your horizon. Because of this, at high or low latitudes, it takes longer for the Sun to pass through each band of twilight. At the poles, civil twilight can occur for two continuous weeks, while at the Equator, where the Sun sets nearly perpendicular to the horizon, the same phenomenon can be as short as twenty minutes.
Longitude EffectsBecause of the Earth's steady rotation, your longitude strongly affects the timing of events such as sunrise and sunset. Although the standard time zones are designed to account for this, they still leave room for significant variations. Depending on how far east or west you are from the central longitude of your time zone, local solar noon will occur before or after 12:00 (noon).
The Earth rotates 360° of longitude every 24 hours.* Dividing 360 by 24, we discover that the Earth rotates 15° every hour indicating that each hour the Sun's position relative to the Earth has moved 15° to the west. This causes the Sun to rise, set and reach its highest point an hour later for every 15° of longitude you move west. Similarly, if you move 15° east, you will observe these phenomena occurring an hour earlier. Larger or smaller angular movements change the timing proportionally such that at 7.5° movement changes the timing by half an hour, etc.
The Standard Time zones are 15°-wide bands of longitude** in which all clocks are set to the same time. To compensate for the effect of Earth's rotation explained above, the clocks in each time zone are set an hour earlier as you move west. As you move east, clocks are set an hour later. Though practical, this means that only the people in the center of each time zone will read 12:00 (noon)* on their clocks at mid-day when the Sun is at its highest point (local solar noon). For the rest of the people in a time zone there is no connection between 12:00 (noon) and the middle of the daylight period. Those on the eastern border of the time zone (7.5° east of the time zone's central longitude) will observe local solar noon at 11:30 and those on the western border (7.5° west of the time zone's central longitude) will observe the same phenomenon at 12:30. Because countries like China, which spans roughly 60° of longitude, observe one time throughout the entire country, the discrepancies between 12:00 (noon) and local solar noon can become even more extreme.
By shifting local solar noon relative to 12:00 noon, your longitudinal position with respect to your time zone's central longitude (meridian) determines the overall rotation of each daylight diagram in the Yearlight Calendar. As stated above, on the meridian of each time zone, local solar noon occurs, on average, when clocks read 12:00 (noon)*. This results in daylight diagrams that are symmetric about the vertical axis. As you move east of a specific time zone's center, the diagram for each day will rotate counter-clockwise, indicating that the Sun appears overhead before your clock reads 12:00. Alternatively if you move west of your time zone's meridian the diagrams will rotate clockwise, indicating that the Sun appears overhead after your clock reads 12:00.
The rotation of the diagrams is additionally impacted by periods of daylights savings time. In regions where daylight savings time is recognized, the diagram will rotate clockwise 15° (the equivalent of one hour) while daylight savings time is in effect.
* Note: slight variations occur due to the fact that Earth's orbit is not a perfect circle but rather an ellipse.
** Note: For political reasons, time zones' borders stray widely from the 15°-wide bands of longitude described here. These discrepancies increase the variations possible within the Yearlight Calendar.
MOON, EARTH, SUN, AND LOCATIONThe Moon phenomena depicted by the Yearlight Calendar are a result of both the Moon's position relative to your horizon as well as the relative positions of the Earth, Moon, and Sun. Like the Sun, the Moon's rise and set times are dictated by the relationship between your horizon (your location) and the Moon. For the most part this timing follows the same location-dependent rules as the Sun except that moving 15° changes the moonrise time by roughly an hour and three minutes instead of one hour.* The relationship between the Earth, Sun, and Moon dictates the phase of the Moon as described below.
Earth, Sun, and MoonThe phase of the Moon describes how much of the Moon's surface is visible from Earth. Like the Earth, the Moon does not emit light so only half of its surface is illuminated by the Sun at any given time.** How much of this illuminated surface we can see from Earth depends on the relative positions of the Earth, Moon, and Sun.
During a Full moon, we can see the entire surface of the Moon illuminated by the Sun because the Earth is between the Moon and Sun. This means that the lit half of the Moon directly faces Earth.
When the Moon, Earth, and Sun form a 90° angle, only half of the Moon's surface is visible to us (half moon). This is because the shadow line separating the day and night sides of the Moon points directly towards Earth.
During a new Moon, there is no Moon visible in the nighttime sky because the Moon is between the Sun and Earth. In this position the illuminated half of the Moon faces away from Earth and is therefore not visible from the Earth's surface.
In the Yearlight Calendar, moonrise and moonset times are plotted in the same way as sunrise and sunset except that the white arc connecting the moonrise and moonset has a varying opacity. The phase of the moon is represented by the opacity of the white line running between moonrise and moonset, from full moon, depicted by a 100% opaque line, to new moon, depicted by a transparent (0% opaque) line. This allows the calendar to depict how much moonlight is in the sky each night.
* This differs from the Sun's time dependence because the Moon orbits the Earth and therefore moves in the sky relative to the Sun.
** Contrary to popular belief, there is no such thing as a "dark side of the Moon". Every location on the Moon experiences both day and night periods.
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