Starting from the upper left and proceeding clockwise, the beginning of spring, today, then summer, autumn and winter. Solar illumination of the earth at the beginning of each of the seasons.
In general then, it is the response of the earth and it’s atmosphere to the changing energy received from the sun. This is what happens on a grand scale as the sun’s altitude changes from season to season as it orbits the sun. Note that the beam from the overhead position is round and concentrated on the surface and the beam from the low angle is elongated and spread out on the surface. A simple thought experiment helps to understand this imagine shining a flashlight on a flat surface, first directly over head and then at a very low angle. In the warmer months, the sun is at its highest in the sky during the coolest months, the sun is at its lowest point in the sky. Since the earth is tilted and its axis always points to the Celestial Poles, the altitude of the sun varies from day to day and from season to season. So, contrary to what some may think, if its not the distance from the sun that is driving the seasonal changes, what is it?
It should also be noted that, paradoxically, the warmest season in the northern hemisphere, the summer, occurs at the earth’s aphelion, the point in the earth’s orbit that is farthest from the sun.
Those points are the NCP (the North Celestial Pole) and the SCP (the South Celestial Pole), the imaginary points on the sky that are the extensions of the earth’s axis. Why, you may ask, do the seasons change and what does it mean to say “ the astronomical beginning of spring“, summer or any of the other seasons? Before we answer that question though, we must first consider that the earth is tilted by approximately 23.5 degrees from the vertical (or 66.5 degrees from the plane of the ecliptic), that it orbits the sun (as all objects do) in an ellipse and that its axis always points to the same points on the sky. Although there is nothing to “see”, per se, this year or any year, it is noteworthy in that it occurs during the nighttime hours since it involves the position of the sun on the ecliptic relative to the Celestial Equator, the imaginary path described by the projection of the earth’s equator onto the sky. For those who reside in the US Northeast, that would be 00:30 (12:30 AM) EDT. This year’s astronomical beginning of spring, the first of two moments during the year when there are precisely 12 hours of daylight and darkness, occurs today at 04:30 Coordinated Universal Time (UTC). Note the waning gibbous moon high in the southwest along with Jupiter, both close to the Ecliptic as expected. Note that the sun is below the eastern horizon with its coordinates illustrated in the upper left corner. Note that this occurs at 30 minutes after midnight (04:30 UTC), today, 20 March for observers on the Eastern Seaboard of the North America. The sky facing south as seen from the US Northeast at the moment of astronomical beginning of spring, 2016. Note the position of the sun on the ecliptic (red grid) relative to the celestial equator (blue grid). The earth as observed from high orbit and facing sunward.