Equinox Daylight and Twilight: Understanding Earths Atmospheres Role
Equinox Daylight and Twilight: Understanding Earth's Atmosphere's Role
On the day of an equinox, the question often arises whether the length of daylight and nighttime are precisely equal, both at the equator and all over the planet. While the concept of equal daylight and nighttime seems straightforward, numerous atmospheric and observational factors contribute to the nuances that make this a complex reality.
Theoretical Equinox Conditions
First, let us consider the ideal scenario where these conditions would hold true. For the length of day and night to be equal, three conditions must be met:
Earth has no atmosphere. Hours of sunrise and sunset are measured from the center of the sun to the horizon. The sun's declination is identically zero for the entire day.Under these ideal conditions, the length of day and night would indeed be equal. However, in reality, Earth's atmosphere and the physical properties of the sun significantly affect these observations.
Atmospheric Refraction and Twilight
Realistically, the longer daylight and nighttime observed on equinox days can be attributed to two main factors: atmospheric refraction and the period of twilight.
Atmospheric Refraction: The sun appears slightly flattened and wider than it is tall when setting or rising due to atmospheric refraction, or the bending of sunlight. This phenomenon becomes more pronounced closer to the horizon, as the light travels horizontally through a greater amount of air.
According to astronomical observations, the sun's light can still reach an observer's eyes for a couple of minutes even after it appears to have set. This means that the "daylight" period is longer than 12 hours. The exact duration varies, with a typical value of about 12 hours and 7 minutes over most of the Earth, increasing slightly as one approaches the poles.
twilight: There is also a period of twilight before sunrise and after sunset due to the scattering of light by the atmosphere. Twilight consists of two distinct parts: civil twilight (when the sun is between 0 to 6 degrees below the horizon), nautical twilight (between 6 to 12 degrees), and astronomical twilight (between 12 to 18 degrees).
Equinox Declination and Time Measurement
The equinox itself is a point in time, not an entire day, which means the sun's declination (the angular distance of the sun from the celestial equator) is only zero for an instant. Even if the day is precisely an equinox, the sun would still have a slight northward or southward movement at 23.4 arcminutes per day, making the exact moment of zero declination difficult to pinpoint.
Furthermore, the center of the sun is about 50 arcminutes below the horizon at sunrise and sunset due to atmospheric refraction. This contributes to the perceived extension of daylight, adding about 3.3 minutes at the equator and 4.7 minutes at 45 degrees latitude. The sun would be above the horizon at both poles, making the day and night equal but still subject to atmospheric conditions.
Conclusion
While the length of daylight and nighttime are very close to being equal on the equinox, atmospheric conditions like refraction and the period of twilight cause minor deviations from the ideal scenario. These deviations are not substantial but are measurable with highly accurate clocks.
Understanding these factors helps us appreciate the complex interplay of Earth's atmosphere and celestial phenomena. It also emphasizes the importance of precise measurements and observations in astronomical studies.