The 2017 equinox length, referring to the precise duration of daylight on the days the sun crosses the celestial equator, represents a fascinating intersection of astronomy, geography, and timekeeping. While the equinoxes are popularly described as the only two days of the year when day and night are exactly equal, this is a pleasant astronomical myth. In reality, atmospheric refraction and the definition of sunrise and sunset create a discrepancy, resulting in the day being several minutes longer than the night. Examining the specific length of the 2017 equinox days provides a concrete example of these astronomical nuances in action.
Understanding the Mechanics of an Equinox
To appreciate the length of the 2017 equinox, one must first understand the mechanics behind the event. An equinox occurs when the center of the sun crosses the plane of the Earth's equator. This astronomical moment happens twice yearly, around March 20 or 21 (the vernal equinox in the Northern Hemisphere) and September 22 or 23 (the autumnal equinox). During this crossing, the subsolar point—the place on Earth where the sun is directly overhead at solar noon—lies on the equator. Consequently, geometrically, every location on Earth should theoretically experience exactly 12 hours of daylight and 12 hours of darkness.
The Role of Atmospheric Refraction
In practice, the geometry of the heavens does not translate to a perfect 12-hour day on the ground. The Earth's atmosphere acts as a lens, bending sunlight as it passes through the air above the horizon. This phenomenon, known as atmospheric refraction, allows us to see the sun before it geometrically rises above the horizon and after it has set geometrically. This "extra" sunlight in the morning and evening effectively lengthens the day. Consequently, even on the day of the equinox, most locations on Earth experience a day length that exceeds 12 hours, a period astronomers call the "equation of time" for diurnal inequality.
The Specifics of the 2017 March Equinox
Turning to the specifics of the 2017 equinox, the astronomical event occurred on March 20, 2017, at 10:28 Coordinated Universal Time (UTC). For observers in the mid-latitudes of the Northern Hemisphere, this vernal equinox signaled the official start of spring. The length of the daylight period varied significantly depending on the observer's latitude. For instance, a city like New York experienced a day length of approximately 12 hours and 8 minutes. In contrast, a location closer to the equator, such as Quito, Ecuador, would have seen a duration much closer to, but still slightly exceeding, 12 hours due to its geographic position.
High Latitudes and the Midnight Sun Effect
The latitude of a location dramatically alters the experience of the equinox. As one moves toward the polar regions, the duration of the day on the equinox increases noticeably. In Anchorage, Alaska, the 2017 March equinox produced a day length of over 12 hours and 40 minutes. This elongation of the day is a precursor to the extreme phenomenon of the midnight sun, which occurs later in the summer for regions within the Arctic Circle. The opposite effect is observed in the Southern Hemisphere, where the same March equinox marked the beginning of autumn, resulting in days slightly shorter than those in the north due to the Earth's orbital eccentricity.
Global Variations and the September Equinox
More perspective on 2017 Equinox length can make the topic easier to follow by connecting earlier points with a few simple takeaways.
