# Why is the solstice the first day of summer, and not the central day?

Summer in the Northern Hemisphere starts on the day of the summer solstice. This is the day that the Northern Hemisphere receives more light from the Sun, due to the Earth's tilt. To my knowledge, the amount of light we receive is related to the temperatures that we have. That's why summer is the warmest period in most regions of the Northern Hemisphere.

Then, shouldn't the solstice be the central day of the summer instead of the initial day? That way, summer would be constituted by the days of the year when more light is received in the Northern Hemisphere, which is related to the higher temperatures in practice.

• I'm not sure how it works elsewhere, but throughout most of the US, the coldest months are Jan and Feb, and the warmest July, Aug, which are the months between the solstices and equinoxes. So, I think it does make sense from that point of view. Feb 4 at 20:36
• As an amateur observer, I'd think that the mechanism is that it takes some time for things to warm up... or cool off ... Feb 4 at 21:05
• en.wikipedia.org/wiki/Midsummer Feb 4 at 22:49
• Seems there are different definitions. The Wikipedia entry for Summer says it's centered on the solstice. Merriam-Webster says it's June, Jul, Aug or from solstice to equinox. Feb 4 at 23:32
• Summer in the northern hemisphere starts on the day of the summer solstice. - According to whom? Feb 5 at 13:59

The English word "summer" means the season of the year that is associated with higher temperatures and shorter nights. There is no official "first day of summer" and different groups of people take different conventions.

One possible convention is to take "June, July and August" as summer, so the first day of summer is June 1st. This is the convention taken by the Met Office in the UK, and roughly corresponds to the warmest temperatures in the UK.

(The reason that the warmest temperatures are not around the solstice is nothing to do with astronomy, it is because the surface takes some time to warm up, so there is a lag between the longest day and the highest temperature)

Another possible convention is to take June 21st to Sept 20th as "summer". This fits the solstice and equinox and still roughly corresponds to the warmer days of the year in the Northern Hemisphere. This is the convention in many modern calendars.

Another possibility is to take the "cross-quarter days" (named in Gaelic Samhain, Imbolc, Beltane, and Lughnasadh) So summer would be from Beltane/May day to Lughnasadh/Lammas day: May 1st - August 1st. This matches the shortest nights, but generally, May is cooler than August in the UK, so is not consistent with summer meaning "warmest season of the year".

The big point here is that, there is no official definition of summer.

• For context, here in Australia I’ve always used and understood “summer” to mean December, January, and February, at least when it refers to a bounded time period. To my recollection, it wasn’t until I was an adult that I encountered alternative definitions, or realised in hindsight that some past encounters with the term may not have meant what I’d assumed! (And then on some website or other I got downvoted for disagreeing with the solstice-to-equinox definition…) Feb 5 at 7:08
• Don't forget the US "unofficial summer", which runs from Memorial Day (last Monday in May) to Labor Day (first Monday in September). Feb 5 at 20:36
• @TimPederick that's very strange indeed, when I first moved to Australia I found it (using the 1st day of the month) very confusing... and the BOM still uses it: bom.gov.au/climate/glossary/seasons.shtml Feb 6 at 3:52
• In Spanish class in the far north of Sweden (Kiruna), we had a great laugh when a student listed the months of winter in Spanish: octubre, noviembre, diciembre, enero, febrero, marzo. Another definition (in use by the Swedish meteorological institude based on WMO definitions) is any month with the 24-hour average temperature below 0°C (by this definition, the UK has no winter except for parts of the Scottish Highlands). Feb 6 at 7:48
• Historically, farming activity has been important in use of the names of the seasons. With the Irish calendar definitions, the various activities tend to fit with the definitions and places the solstices and equinoxes pretty centrally within their seasons. Feb 6 at 12:53

As others have suggested, the definition of when seasons change is arbitrary. The advantage that using solstices and equinoxes as the dividing points of the seasons is that it's easy to determine these precise dates. They correspond to specific astronomical phenomena that can be measured and predicted.

If you call them the middles of the seasons, then how do you determine which day each season begins? You could use the halfway point between each pair of solstice and equinox, but this feels less meaningful to astronomers. Earth's motion around the Sun isn't uniform, so this halfway point doesn't correspond to anything actually happening physically.

• On the flip side, I think a "new moon" is often defined as the midpoint between the time the moon ceases to be visible and the time it becomes visible again, since judgments of when those events occur may depend upon the sensitivity of the observer, but an observer that perceives the moon as becoming invisible sooner would perceive it as becoming visible later. Feb 6 at 15:57
• I was thinking about that analogy. While it does depend on the observer's sensitivity, it's still easier than trying to determine the precise day of a full moon. Feb 6 at 16:00

A day of just the right length will warm the Northern hemisphere just as much as it cools over the following night, as measured by thermal Joules. The summer solstice is longer than that, or else a year would see us cool; by the same logic, this sweet spot is also longer than the winter solstice. After the summer solstice, the days shorten, but remain long for a while, so warming continues. The NH's hottest time of year is therefore quite a while afterward; there is a similar delay in comparing the coldest time to the darkest.