The difference between the actual apparent solar time and the “expected” mean solar time is described by the equation of time. The Sun isn’t always at its highest point in the sky at 12:00 PM 12:00 local mean solar time, sometimes it’s a bit ahead of time and sometimes it’s late. The Sun may seem tiny, but its angular diameter is only 0.53°, while the entire analemma spans 2 × 23.437° = 46.874° in the vertical direction. In the simulation below you can observe the position of the Earth and its axis of rotation at different points of the year: If you were to look at the orbital plane from above you’d notice that the axial tilt of the Earth isn’t aligned to either the major or the minor axis of the orbit’s ellipse. The actual duration of each individual day varies, but before we witness that variation we have to discuss the most important consequence of the axial tilt. In fact, 24 hours is the duration of a mean solar day. If you look closely at the simulation of the sidereal and solar day you’ll notice that we didn’t account for two important factors – eccentricity of the orbit and the axial tilt of the Earth. The duration between two solar noons is known as a solar day which lasts the familiar 24 hours. Despite heavily irregular shapes the centers of the time zones correspond to roughly noon of local solar time. Instead, time zones are used to provide a more uniform timing within different regions of the world. Naturally, we don’t set our clocks to solar time. Noon is the time of day at which the Sun is at the highest point in the sky which also means that the shadows cast by the sunlight are the shortest. In Latin ante meridiem means before midday, while post meridiem is after midday. This is also where the terms AM and PM come from. As such, the apparent Sun size is only 3.3% smaller when the Sun is the farthest from Earth compared to when it’s closest:ĭepending on one’s position on the Earth, during a local solar noon the subsolar point is either directly north, directly south, or directly at the current location. The eccentricity of the Earth’s orbit is fairly small so the distance between the center of the Sun at aphelion ( 94,509,460 mi 152,098,233 km) is not that much larger than that at perihelion ( 91,402,640 mi 147,098,291 km). A sidereal year is actually not the basis of year tracking in civilian time, but we’ll have to wait until the later parts of this article to discuss the period of Earth’s journey around the Sun that defines how we count years. A sidereal year lasts roughly 365.256 days. The time it takes Earth to get back to the same point on the orbit is called a sidereal year where sidereal means “with respect to stars”. We’ll soon see that there are multiple ways to define “a day”, but unless stated otherwise we’ll assume that it lasts 24 hours, that is 24 × 60 × 60 = 86400 seconds. If you pay a close attention to the position of the Earth at the beginning of a calendar year and compare it with the beginning of the next year you’ll notice that the Earth hasn’t returned to the exact same position – during 365 days the Earth falls a little short of the full orbit. Its eccentricity is equal to 0.0167 and the Sun is very close to the center: You may also be barely able to tell that the orbit is elliptical. At your current viewing scale the Earth is pretty much invisible, it has a diameter of pixels, so instead its location is shown by an arrow. The tiny yellow dot in the middle depicts the Sun. Below you’ll find a visualization of the Earth’s orbit around the Sun with proper scales. What we’ve seen above paints a convenient picture, but the actual relative sizes are very different. However, the Sun’s mass is so dominant that the actual focal point is fairly close to the Sun’s center. Technically, the Sun is not in one of the focal points of the ellipse, but instead the objects in the Solar System revolve around its center of mass. There are two important points on the orbit: the perihelion is the point at which Earth is closest to the Sun, while at the aphelion Earth is the furthest away from the Sun. The small cross in the middle shows the center of the ellipse while the small rings, one of which is directly in the center of the Sun, symbolize the focal points.
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