Welcome to the October night sky. On the last Sunday of this month the clocks go back an hour so it gets darker earlier, this puts Portugal on to universal time (UTC).
This is the same as the old Greenwich Mean Time (GMT), and what this means is the clocks from now on should in theory match up with the position of the Sun, so at 12 o’clock midday the Sun should be at its highest point in the sky.
It is not quite as simple as that as this only applies to the zero degree meridian (Greenwich Meridian) and as Portugal is about seven and a half degrees west of this it means that the Sun is a little bit late as regards the clocks.
Just how much later is easy to work out as the Earth rotates 360º in 24 hours relative to the Sun and this means 15º of rotation in one hour of time. So therefore, 7 ½ divided by 15 equals ½ hour or 30 minutes late.
So midday solar is approximately 12:30 pm in Portugal. There are other complications to this as the orbital speed of the Earth varies slightly due to our elliptical orbit.
The practical effect of this is that sundials are not always correct; in fact the Sun can be more than 15 minutes fast in November and nearly 15 minutes slow in February.
As long as you remember to correct for the offset from the Greenwich Meridian (approximately 30 minutes slow in Portugal) sundials read correctly only in the middle of April, in the middle of June, the beginning of September and Christmas day!
The month of October also marks the opposition of the gas giant planet Jupiter. This planet is in the constellation of Aries and rises in the east at sunset and is visible throughout the hours of darkness.
The full Moon is close to Jupiter on the nights of 12th and 13th.
The Orionids meteor shower should be visible on 21st and these fast yellowish coloured meteors emanate from the direction of the constellation of Orion that can be seen rising in the east 2 hours after Jupiter has risen.
These meteors are dust left over from the tail of Halley’s Comet.
The Moon is first quarter on the 4th, full on the 12th and last quarter on the 20th and new again on the 26th of October.
Clive Jackson can be contacted on tel. 281 322 527, fax 281 321 754 or email cdepa@mail.telepac.pt. The web address is www.cdepa.pt.
