The arrival of third week in June brings us the last day of spring and the first day of summer. This year, it will occur at 3:07am (PST on the west coast, or 10:07am GMT) on Thursday, June 21, but only in the Earth’s northern hemisphere. In the southern hemisphere, it’s the winter solstice.
But how can we be so precise? How can we know that it occurs at 3:07am? The trick is in understanding a bit more about the summer solstice in terms of our planet’s orbit around the sun.
As we travel around the sun, Earth is spinning in a 24-hour long day on its north-south axis. But the north-south axis of the planet has a wobble, so it’s not spinning like a perfect top. Twice a year, on the spring equinox and the fall equinox, it does spin like a perfect top because the equator is the point on Earth nearest the sun, relative to our north-south axis.
Think of the Earth in cross section, imagining the equator around the middle of the planet at latitude 0°, and the north pole at the top, 90° north (with the south pole being 90° south). Because of the wobble in our north-south axis, there is also a wobble at the equator. For half the year, the sun is closer to points north of the equator, and we get summer. For the other half of the year, the planet tilts the other way, and is closer to points south of the equator and we get winter.
The wobble in the Earth’s axis is exactly 23.45°. Summer solstice is the longest day of the year in the northern hemisphere, so the wobble has reached its turning point. After this time, the days will gradually begin getting shorter again. Thankfully, it’s hardly noticeable until around the end of summer. The line of latitude around the Earth that is 23.45° north of the equator is the Tropic of Cancer. The latitude 23.45° south of the equator is the Tropic of Capricorn. This is the point nearest the sun on our winter solstice – the shortest day of the year for us at the north. Not surprisingly, the area between the two tropical lines are known as… the tropics.
But what does this have to do with gardening? Well, for one thing, spring is over. It’s now too late to plant most types of seeds hoping that they will mature during the summer. There are some very fast growing exceptions, of course, like arugula and cress in the vegetable garden, and Alyssum, Gypsophila, and Linum in the flower patch. Now that the summer solstice has arrived, it’s time to think about planting for fall and winter harvests, and for overwintering crops.
Some crops respond to day length in a direct way. While our days in the north get dramatically longer at summer’s arrival (it’s 24 hours of sunshine now for people in the far north), those closer to the equator notice less of a difference in day length. It’s always hot there, and it’s always more or less 12 hours of daylight, 12 hours of night. Northerners must grow “long-day” onions, which respond to day length by bulb formation. Bulbing isn’t triggered in these onions until the days are 14-15 hours long. Farther south, “short-day” onions are grown, and they form bulbs when the days are just 10 hours long.
This is also a transitional point for gardeners, who switch from focusing on cool season crops, from lettuce and spinach, to radishes and peas. Now is the time to focus on heat-loving crops like tomatoes, peppers, and squash. Well prepared gardeners switch tactics with these changes in the seasons in order to make the most of the available heat or coolness.