From the combination of sol (sun) and sistere (cause to stand or stop) the solstice marks the highest or lowest points of the sun’s path in the sky each year.

In summer, for instance, the sun’s arc in the sky stops its daily ascent at the longest day of the year. There it reverses, beginning its descent to the autumnal equinox — with equal day and night — and eventually to its lowest point at the year’s shortest day, the winter solstice. At that point, its descent finished, the sun once again begins its slow climb towards the summer solstice.

Solstice

On the equinox there is nearly exactly 12 hours of daylight and night — the day and night are perfectly balanced. On these days the sun rises almost due East and sets due West. The Spring equinox and Fall equinox are the transition between the seasons of Winter to Spring or Summer to Fall, depending on if your days are getting longer or shorter. The other seasons transition at the solstices.

Technically, the equinox is the instant “the plane of Earth’s equator passes through the center of the Sun.” (wikipedia) This usually happens around the 20 March and 23 September.

The equinox exists due to the tilt of the Earth. See Seasons

Equinox

What a lovely time of year—when the leaves of deciduous trees in many forests change from the deep greens of summer to the yellows, oranges and reds of fall. But what makes the leaves change colour and what compounds are responsible for Nature's spectacular display?

We all know the magical green chlorophyll, but leaves also contain carotenoids. As the chlorophyll fades, the golden oranges of these carotenoids start to shine through. You may recognise one carotenoid, beta-carotene, from carrots. Another carotenoid, xanthophyll, is responsible for the beautiful bright yellows. The leaves of some trees also produce anthocyanin as Autumn begins that gives beautiful deep reds.

Autumn leaves and the compounds that cause their colours

A full moon that hits twice in a calendar month. Given the cycle from full moon to full moon is about 29.5 days it doesn’t happen very often that this coincides so nicely. And February never has a blue moon.

I like this definition, but it is apparently a modification to the original definition which is a little more complicated. Given the 29.5 day cycle, most seasons of 3 months have 3 full moons. But every now and then, a season would come by, like Sep, Oct, Nov in the sketch, where there would be 4 full moons. The 3rd of these was known as a blue moon, or a seasonal blue moon. See why I like the modern definition?

Here’s a good article that explains the subtleties and history of blue moons.

Check out more moon or astronomy sketchplanations

What is a blue moon?

Are caused by the Earth’s tilt. As the Earth goes around the sun, the tilt of Earth’s axis — 23.5 degrees — causes at different times of the year, the two hemispheres, North and South, to be either tilted towards the sun or away from it.

In winter, and closer to the poles, because light from the sun hits at a greater angle, the solar energy gets spread over a larger area. This makes it weaker, and cooler and gives rise to winter.

In summer, and closer to the equator, light from the sun hits more directly and so is concentrated on a smaller area making it stronger.

The tilt also means that the number of daylight hours is greater in the summer than in the winter, and hence the amount of time it gets heated, which also contributes to the seasons.

Seasons are reversed in the two hemispheres, so if it’s winter in the Northern Hemisphere it’s summer in the Southern and vice-versa. The further from the equator you go, the bigger the effect of the seasons. So, temperatures in Ecuador stay relatively constant throughout the year, while the poles experience extremes of 24-hour daylight or 24-hour night and great freezes and great thaws.

Sketchplanations tagged with seasons