With the beautiful weather Friday in Ottawa, you can be forgiven if you forgot that today is the first day of Autumn. Hopefully it’s the first day of a nice long warm fall because I know I’m in absolutely no rush to move into the next season.

So, what makes it the first day of autumn anyway?Astronomically, it’s quite simple: It’s the autumnal equinox (in the northern hemisphere). This means that the tilt of the Earth’s axis is neither towards or away from the Sun. The centre of the Sun is in the same plane as the Earth’s equator. This only happens twice a year — now and the vernal equinox in the Spring. If you were standing on the equator, you’d have been able to look straight up and seen the sun vertically overhead.

This can interfere with people using geostationary satellites because the sun passes behind the satellites from for a short period of time each day starting a couple of days before the equinox and lasting for a couple of days after it. In radio terms, the sun is much louder than the geostationary satellites, so the ground stations can’t hear the satellite. Another way to visualize this is to think about a candle and then put a 150W flood light behind it — you won’t be able to see the flame.

This year, fall started at 09:04 UTC (05:04 Eastern), so it was already fall well before I was awake. 🙂