The First Day of Spring

Eija asked a question in the comments yesterday.  Why does the date of the first day of spring move around from year to year?  Sometimes it is the 20th and sometimes it is the 21st.  Here's why:

From the article, Equinox, Wikipedia 

An equinox in astronomy is the event when the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year. More technically, the equinox happens when the Sun is at one of two opposite points on the celestial sphere where the celestial equator and ecliptic intersect. In a wider sense, the equinoxes are the two days each year when the center of the Sun spends an equal amount of time above and below the horizon at every location on Earth. The word equinox derives from the Latin words aequus (equal) and nox (night)

The two equinoxes can be distinguished by different pairs of names, depending on which feature one wants to stress.

    * Spring equinox (Chunfen) and autumn (Qiufen) or fall equinox. These names can be used when one wants to relate the equinox to a season. The seasons of the northern hemisphere and southern hemisphere are opposites (the spring equinox of one hemisphere is the autumn equinox of the other) so these names can be ambiguous.

    * March equinox and September equinox. An alternative to the previous set, but without the ambiguity for which hemisphere they are intended. These names are still not universal, however, as not all people on Earth use a solar based calendar where the equinoxes occur every year in the same month (they differ in the Jewish calendar, for example). The names are also not useful for other planets (Mars, for example), even though they have seasons.

    * Vernal equinox and autumnal equinox. These names are direct derivatives of Latin (ver = spring, autumnus = autumn), and as such more apt to be found in writings. Although in principle they are subject to the same problem as the spring/autumn names, their use over the centuries has fixed them to the viewpoint of the northern hemisphere. As such the vernal equinox is the equinox where the Sun passes from south to north, and is a zeropoint in some celestial coordinate systems. The name of the other equinox is used less often.

    * The calculation of Easter in the Christian church (first Sunday after the first full moon on or after the March equinox), uses its own definition for the equinox — it always falls on March 21. The earliest possible Easter date in any year is therefore March 22.

One of the effects of equinoctial periods is their temporary disruptive effect on communications satellites. For most geostationary satellites, there is almost always a point when the sun is directly behind the satellite relative to Earth. The Sun's immense power and broad radiation spectrum overload the Earth station's reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit. The duration of those effects varies but can range from an hour to a few minutes.  (Who watched American Idol last night?  I wonder if that is what caused the distruption after LaKisha sang.)

Folk tales from various European countries claim that only on the March equinox day (some may add the September equinox day or may explicitly not), one can balance an egg on its point.  (We'll take pictures if we can get some standing.)

Comments

[eija]

Heh.. I didn't really ask why it keeps changing - I've said to my kids that it does - but why do you say it's today of all days. And just after posting my comment I looked at my calendar and noticed it's the Spring equinox!

For us in Finland the Spring equinox is the day when day and night are equally long. I've never heard that egg thing - sounds a bit funny :D