Spring Equinox

[RTFishall]

I always thought that at the Spring (and Autume) Equinox the lenght of the day and night would be exactly the same. However, I noticed that the published sun/rise set times showed a difference of a few minutes. Why is that?

[DTCwordfan]

because the axis of the earth varies a bit - for example the Japanese earthquake shifted it a few minutes.....this is a natural process and nothing to be alarmed at.

[Azalian]

The days when the day/night split are most equal are called the "equiluxes"
These are actual days not moments of time like the equinoxes.

[wildwood]

Because straight after any Equinox the days start to lengthen/shorten again by a few minutes.

[New Judge]

Yes. The "equinox" is the precise moment when the earth' s axis is perpendicular to the plane of its orbit and is tilted neither towards or away from the sun. The "equilux" is the day around that time when the periods of day and night are as near to equal as is possible.

[jomifl]

To get it right and have equal day / night length sunrise and sunset should be defined as when the centre of the sun touches the horizon, but we use the upper edge, so it isn't symmetrical.

[vascop]

@nj: The earth's axis is never perpendicular to the orbital plane. The plane containing the axis is perpendiculat to the orbital plane at the equinoxes.

[New Judge]

Yes thanks vascop. Of course the earth is always tilted at 23 degrees relative to its orbit. I didn't quite know how to explain that at the equinoxes it is tilted neither away from or to the sun. Thanks.