december 21 - march 20 is the name of the day that winter occurs in the northern hemisphere.
what is the name of the day that winter occurs in the northern hemisphere?
december 21 - march 20
by astronomical convention, the four seasons are determined by the solstices (the two points in the earth's orbit of the maximum tilt of the earth's axis, toward the sun or away from the sun) and the equinoxes (the two points in the earth's orbit where the earth's tilted axis and an imaginary line drawn from the earth to the sun are exactly perpendicular to one another). the solstices and equinoxes divide the year up into four approximately equal parts. in the northern hemisphere winter solstice occurs on or about december 21; summer solstice is near june 21; spring equinox is around march 20; and autumnal equinox is about september 23. the effect of the earth's axial tilt in the southern hemisphere is the opposite of that in the northern hemisphere, thus the seasons of the solstices and equinoxes in the southern hemisphere are the reverse of those in the northern hemisphere (e.g. the northern summer solstice is at the same time as the southern winter solstice).
earth's orbit
perhaps one of the most apparent factors contributing to earth climate change is the angle at which the earth is tilted. this is the angle at which earth's axis of rotation is from the vertical, also known as earth's obliquity. earth's current tilt angle is approximately 23.5 degrees. the axial tilt angle affects climate largely by determining which parts of the earth get more sunlight during different stages of the year. this is the primary cause for the different seasons earth experiences throughout the year, as well as the intensity of the seasons for higher latitudes. for example, in the northern hemisphere, if there were no axial tilt, i.e. earth's obliquity would be zero degrees, then there would be no change in the seasons from year to year. this would be because there would be no difference in the amount of solar irradiation received, year-round, anywhere on earth. on the other hand, if earth's axial tilt angle was great (45+ degrees), the seasonality of each hemisphere, individually, would be highly exaggerated. summers would be extremely hot, with substantially more hours of daylight than night each day. winters would be extremely cold, with substantially more hours of night than daylight each day. this is because, during summer for the northern hemisphere, if the earth is tilted more (pointed towards the sun more), there would be more available hours in which the suns rays can strike any certain place, thereby increasing the number of daylight hours at any given place, with more and more daylight hours at higher latitudes. also, because the northern hemisphere would be tilted much more towards the sun, it would be physically closer to the sun, thereby increasing the intensity of the sun's rays hitting the northern hemisphere, thereby causing the northern hemisphere to become hotter. likewise, during winter for the northern hemisphere, there would be fewer hours of daylight because the northern hemisphere would essentially be pointed away from the sun. fewer daylight hours means less solar radiation hitting the northern hemisphere, especially at higher latitudes, and therefore causing the northern hemisphere to become colder. the same things can also be said about the southern hemisphere, particularly at high latitudes. in either case, the climate around the equator is not affected nearly as much as the higher latitudes, thereby creating a sizable difference in how obliquity affects different latitudes. this is all, of course, dependent on what the actual tilt angle is at any given point in time. the thing is, though, that earth does in fact change obliquity over time in a cyclic pattern. earth's obliquity does not change much, though, as obliquity has been determined to cycle between the small range of 22.2 degrees to 24.5 degrees, in a cycle that lasts approximately 41,000 years. therefore, with the small tilt variation over time, the earth has always been thought to have had a seasonal climate, at least in the high latitudes due to the solar effect of changing earth obliquity.
orbital effects on climate