---
title: 21st century season calendar
date: 2018-04-21 16:58

tags: calendar, solstice, equinox, perihelion, aphelion

links:
  - url: https://gkn.me.uk/seasons/seasons.ics
    title: Solstice, equinox, perihelion and aphelion calendar, 2000 to 2100
    rel: related
    type: text/html
  - url: webcal://gkn.me.uk/seasons/seasons.ics
    title: Subscribe to the calendar
    rel: related
    type: text/html

---

I like to be aware of the solstices and equinoxes,
because they're the closest things we have to natural holidays,
so I want to have them as events in my calendar.

I haven't been able to find a calendar to subscribe to
in iCalendar format,
that shows the solstices and equinoxes
without a load of extra messy stuff,
and doesn't expire in the next few years.

Luckily,
the Astronomical Applications Department of the United States Naval Observatory
generously makes a lot of astronomical data available
[in such a way that it can be gathered and manipulated easily][api],
with [free permission to reuse the data][reuse].

[api]: http://aa.usno.navy.mil/data/docs/api.php#seasons
[reuse]: http://aa.usno.navy.mil/about/staff/docs/PrivacyNotice.php "“All information on this site is considered public information and may be distributed or copied unless otherwise specified.”"

So I've downloaded the data for the entire 21st century,
and converted it into iCalendar format.
It's a simple plain text format,
so I was able to do this manually with plenty of find-and-replace.

The result is a calendar with 6 events per year:

* the two solstices in June and December
* the two equinoxes in March and September
* the perihelion in January
* the aphelion in July

To subscribe or import the data, point your calendar program at
[https://gkn.me.uk/seasons/seasons.ics](/seasons/seasons.ics).
Or, going to [webcal://gkn.me.uk/seasons/seasons.ics](webcal://gkn.me.uk/seasons/seasons.ics)
may prompt your web browser to send this URL to your calendar program,
depending on how co-operative all the software on your computer is.

The calendar's contents won't change:
it's unlikely that new observations would make the data obsolete
(perhaps an inconsequential minute or two either way)
and I don't expect to have any personal use for data beyond 2100.
Someone else will have to fulfil your 22nd-century calendar needs.