Sunrise, Sunset, and Solar Time
The alternation between day and night and the variation in daylight hours are fundamental phenomena in navigation and daily life. These events are not random, but depend on the geometry of the Earth and its movement around the Sun. Below, we explore the key concepts for understanding how they work.
Seasons and Earth Movement
The main cause of seasonal changes is the tilt of the Earth's axis (approximately 23.5°) with respect to its orbit around the Sun. This tilt causes sunlight to fall differently depending on the time of year and latitude.
- Solstices: They are the moments when the Sun reaches its highest or lowest point in the sky. At the June solstice, the Northern Hemisphere tilts toward the Sun, causing summer (long days) in the north and winter in the south. At the December solstice, the opposite occurs: it is summer in the Southern Hemisphere and the Sun reaches its highest point there.
- Equinoxes: They occur in March and September. On these dates, the duration of day and night is practically equal throughout the planet, as the Sun is located directly over the Equator.
Duration of Daylight
The amount of daylight varies dramatically depending on where we are:
- At the Equator: Days and nights last approximately 12 hours throughout the year.
- At high latitudes: As we move away from the Equator toward the poles, seasonal differences become exaggerated. In summer, days are very long (reaching 24 hours of light during the Midnight Sun within the polar circles). In winter, days shorten until reaching total darkness or Polar Night.
Time Measurement: LMT and UTC
For navigation, it is crucial to distinguish between different types of time:
- Local Mean Time (LMT): It is based on the position of the Sun relative to a specific meridian. Local noon occurs when the Sun is at its highest point over that meridian. All places at the same longitude have the same LMT.
- Coordinated Universal Time (UTC): It is the global standard used in aviation to avoid confusion. It is based on the Greenwich meridian. As the Earth rotates from West to East, when moving eastward, local time is more advanced than UTC; westward, it is earlier.
| Arc to Time Conversion | Value |
|---|---|
| $360^\circ \text{ of longitude}$ | $24 \text{ hours}$ |
| $15^\circ \text{ of longitude}$ | $1 \text{ hour}$ |
| $1^\circ \text{ of longitude}$ | $4 \text{ minutes}$ |
Sunrise, Sunset and Twilight
Technically, sunrise occurs when the upper edge of the Sun appears on the horizon, and sunset when it disappears. However, there is light before and after these moments due to the atmosphere.
- Civil Twilight: It is the period before sunrise or after sunset when the center of the Sun is up to 6° below the horizon. During this time, there is generally enough light to see without artificial lighting.
- Twilight Duration: It depends on latitude and season. At high latitudes, the Sun sets at a shallow angle, which makes twilight last much longer than at the Equator, where the Sun "falls" almost vertically.
Factors Affecting Visibility
Two important phenomena alter when we actually see the Sun:
- Atmospheric Refraction: The atmosphere acts as a lens that bends light. This allows us to see the Sun a few minutes before it geometrically rises above the horizon and a few minutes after it has set.
- Observer Altitude: At greater height, the visible horizon is farther away and "lower". This means that, if you are flying high or on a mountain, you will see sunrise earlier and sunset later than someone who is at sea level.