Before anything else it's important to be aware of some things:
- Planet Earth its not a perfect sphere, its flattened on the poles;
- There is an imaginary rotation axis that passes on these poles;
- this axis, where the planet rotates has a inclination of about 23,45º,
- this angle varied over the planet history;
- Planet Earth moves in tow distinct ways:
- The translatory motion, around the Sun,
- The rotation motion, around the imaginary axis.
- Planet Earth describes a elliptic orbit around the Sun, this means the orbit its not perfectly round, that means the Earth is closer to the sun in certain times.
- Seasons are the result of the axis inclination.
The quick answer to the question "Why do we have winter and summer?" is the one described on the topic number 5., "seasons exist because the planet is closer/further from the Sun", WRONG, simply think a little and you will see this is not true. If this was true ALL the planet would be at same time in the summer or in the winter.
Let's say we are in North hemisphere, above the equator. If someone asks us about the seasons we will say March for spring, August for summer, autumn to Halloween, and of course winter for Christmas. However we don't think "why?” we don’t even think that is not true for everyone. In Brazil for example Christmas is summer, and autumn is March. While North hemisphere fills the beach, South hemisphere prepares for the winter.
Reasons for this are caused by the 23º inclination of the imaginary earth axis.
To understand this we must to be able to place ourselves outside the planet, in front of the Sun, observing the Earth "walking" around the star. Like the image shows the Earth turns around himself (rotation) and at same time moves around the Sun. Due to the 23,45º inclination of the Earth the planet orientation changes relative to the Sun along the year. This makes the length of the days to change along the year.
Now, imagine yourself in North hemisphere, looking to the draw is easy to understand that when the North Hemisphere moves away from the Sun, South Hemisphere approaches it (as if the planet is leaning back), we also note that when the planet is farthest from the Sun coincides with Winter in the North and summer in the South. Using the same logic we can say the inverse is also true.
Place yourself again in the planet, anywhere. Look at the Sun; what is the arc the Sun travels? During a whole year the path the star travels in the sky varies:
In the winter the path is shorter, and closer to horizon, and therefore takes less time to travel the distance;
In the summer the path is bigger, and far from the horizon, and therefore takes more time to travel the distance.
So, Debbie, the result of this is that in the winter the days are shorter and in the summer the days are longer, this variation is progressive like almost everything in Nature, therefore in autumn the days start to get shorter and in spring longest.
Equinoxes- day is exactly equal to the night. In the North Hemisphere the Spring Equinox occurs in March, and the autumn Equinox in September.
20 or 21 March and 22 or 23 September
Solstices- when occurs in summer means that the length of daylight is the longest of the year. Similarly, when it occurs in winter, means that the length of the night is the longest of the year.
20 or 21 and June 21 or 22 December