As you know, the shortest day in the northern hemisphere falls on December 22. However, the latest sunrise at the latitude of Moscow occurs near the New Year. What is the reason for the apparent discrepancy? How will the date of the latest sunrise change when moving north and south?
Due to the ellipticity of the Earth’s orbit and the tilt of its equator to the ecliptic, the gap between the two subsequent upper climaxes of the Sun does not remain constant. True solar noon occurs at 12h + n local time, where the value of n is called the equation of time. In late December and early January, near the winter solstice and the Earth’s orbital perihelion point, the angular velocity of the Earth’s rotation around the Sun exceeds the average for the year, in addition, the right ascension of the Sun increases faster due to its movement far from the celestial equator. As a result, the interval between two noon days is about half a minute greater than 24 hours, and the equation of time is rapidly increasing, which is why each day both sunrise and sunset occur slightly later than the previous day.
This effect is especially noticeable in tropical latitudes, where declination changes have less impact on sunrise and sunset times. There, the date of the latest sunrise is shifted to even later dates in January, and near the equator – even to February – the epoch of the maximum value z). Near the Arctic Circle, where the Sun only briefly appears above the horizon, abrupt changes in the length of the day lead to the fact that the date of the latest sunrise approaches the day of the winter solstice – December 22.
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