Time

Solar time

I am sometimes asked why I stick to the old "GMT" instead of adopting the more commonly used UT (Universal Time) or UTC (Coordinated Universal Time). For all practical purposes the three are synonymous, although their derivations differ in detail. The truth is that I have an affection for the Greenwich meridian having worked on transit and meridian circles (two instruments used for positional work). Longitude is still established from the Greenwich meridian.

Due to irregularities in the Earth's rotation there will inevitably be a small discrepancy between the time interval marked out by atomic clocks and the time calibrated from the Earth's rotation. The "mean" in GMT derives from the fact that noon is determined not by the transit of the real Sun upon the meridian but by a "derived" sun, called the mean sun. This is because the interval between successive transits of the real Sun over a given meridian alters day by day due to a number of factors (including the Earth's varying speed in its orbit and what is called the obliquity of the ecliptic). Deriving a convenient time base for civil purposes compensates for these irregularities by the introduction of the "equation of time" giving us the mean sun. In our calculations the mean sun and the real Sun come into synchronism on two days of each year.

The clocks we use are set to a given meridian convenient for general purposes. True noon is the instant at which the Sun in the sky (the real Sun) transits the local meridian and is specific to the observer's position. Noon as registered by a clock keeping GMT is the instant at which the mean sun transits the Greenwich meridian. A difference in longitude of 15 degrees between one station and another is equivalent to a difference in time for the two stations of one hour. Those living west of the Greenwich meridian (Orkney lies between longitude 2º 20' and 3º 28' west), and using GMT (UT), experience slightly darker mornings and lighter evenings according to the clock. In large countries (as indeed across continents) it is necessary to have time zones in which clocks are set to accommodate the significant differences in longitude. In using a sundial one has to take into account all the above factors.

It used to be considered reasonable to have clocks registering noon close to the middle of the day itself giving about equal amounts of daylight either side. The introduction of summer time (and in some countries, double summer time) where the clocks are advanced on "mean time" by one or two hours, could be seen as a fad from the astronomer's perspective. The apparent gain in light of an evening could be achieved by starting the day earlier - having the "working day" symmetrical about noon, commencing at eight and ending at four.

Most of us are aware of the leap year used to bring the days into line on the calendar; fewer people are aware of the calculations necessary to give us a daily time base that will not get out of kilter and disrupt our ordinary domestic and commercial affairs.

Sidereal time

The day, as we have seen, is related to the rotation of the Earth upon its axis. The mean solar day measures the interval between successive transits over the same meridian by the mean sun. In common with the true or real Sun, the mean sun moves from west to east against the background of stars at the rate of a little under 1 arc degree per day. Therefore, if the day were to be reckoned as the successive transits of a star we would have a closer approximation to the true rotation of the Earth, assuming the star to be fixed in position. This we might call the sidereal day.

Astronomers use the concept of a sidereal day based upon the successive transits of a position on the celestial sphere called the First Point of Aries. This is defined as the intersection of the celestial equator with the ecliptic. This point is also used as the zero of Right Ascension (RA), as described in a previous article. Supposing all the stars to be sensibly fixed in position relative to one another we find that the First Point of Aries is itself making a steady progression against the background of stars resulting in a complete circuit of the heavens over a period of approximately 25,800 years. This is caused by a slow wobble or gyration of the Earth's axis and is referred to as precession. It is because when this reference was first established it fell within the constellation Aries that it retains the name despite now lying in the constellation Pisces. The displacement of the position of the First Point of Aries with time is called the precession of the equinox.

The Mean Sidereal day is defined as the interval between two successive transits over a meridian of the First Point of Aries. The sidereal day, therefore, is shorter than the solar day by about 4 minutes and this is reflected in the earlier rising of a star day by day of about that order. (For example on January 1st 2004 the bright star Arcturus rises at 23h 12.5m, a week later it rises at 22h 45.1.)

A clock keeping sidereal time is standard equipment in most professional observatories.

The nature of time

The concept of time in the broader sense has and will occupy our contemplation for as long as there are minds to do so (note, I do NOT say to the end of time). It is by no means a preoccupation of modern cosmologists - philosophers as far back as Plato and beyond had profound things to say on the issue. Of course we expect with all the advances in contemporary science to be in a better position to formulate our understanding of time. In his best selling book "A brief history of time" Stephen Hawking writes on page one: "Only time (whatever that might be) will tell." The bracketed words are crucial here, surely?

The following philosophical conundrum, or tease, might be worth a little thought: "Time is the vessel of deeds - no deed done, no time gone?" * (A recipe for idleness?)

For those apparently wanting a temporary relief from boredom, and who suggest a time reversal (or something of the sort), I offer:

"To escape time and cheat the bell, were that possible could I tell?" *

[* From "A book of aphorisms" by the writer, 1992.]

JV 23/06/03

Twilight and the Noctilucent clouds

Fig 1: Noctilucent clouds, Rousay, July 1999 photo: J V 24mm Nikkor f2.8, 4.5 secs. Fujicolor 400 ASA

We would draw attention to the archives and the short item on twilight.

Civil Twilight. Twilight is said to be in effect once the Sun is below the horizon. The instant at which the Sun's upper limb disappears (at sunset) signals the commencement of civil twilight which then lasts until the Sun's centre is 6º below the horizon.

From this it may be shown that for there to be civil twilight at midnight in the northern hemisphere one must be at a latitude of no less than 60º.6. This rules out Orkney but includes Shetland from Fetlar northward.

Folk in Orkney who have tried the experiment will know that it is quite feasible to read the headlines of a newspaper on a clear night at midnight close to the solstice, when the Sun may be no more than 8º below the northern horizon.

Light pollution notwithstanding, it is an interesting exercise during the summer months on a clear, moonless night to attempt to record the faintest stars visible to the naked eye at midnight or thereabouts.

Fig 2: Noctilucent clouds, Rousay, July 1999 photo: J V 24mm Nikkor f2.8, 4.5 secs. Fujicolor 400 ASA

Noctilucent Clouds

Noctilucent clouds form at heights of around 80 to 85 km, considerably heigher than cirrus clouds which they may seem to resemble in some respects. Once seen and identified the astute observer is not likely to confuse the two. (See figs. 1 & 2)

Noctilucent clouds are most likely to be observed at latitudes of 60º to 80º for some weeks close to summer solstice (June 21 in 2003). They most commonly become visible during lows in the solar cycle. (We are currently witnessing a decline in solar activity from a recent maximum.)

Photographs of noctilucent clouds lasting a few seconds will often show stars to magnitude 2 or fainter. The rule of thumb for identifying these clouds is frequently taken to include a time after sunset at which stars are visible to the naked eye. However, caution is needed before jumping to conclusions. The bright stars, Arcturus, Vega and Capella are quite readily visible to the eye at times of strong twilight when ordinary cirrus clouds are also observable.

Spreading aircraft condensation trails may sometimes simulate noctilucent clouds at elevations in the atmosphere compatible with naturally forming cirrus. On the other hand, correlations between noctilucent clouds (at the much greater height of to 80 km) and aircraft trails formed earlier indicate that these trails may be precursoral to the formation of noctilucent clouds. An even closer relationship is shown where rockets traversing the higher atmosphere produce "artificial" noctilucent clouds.

It would appear, therefore, that for all their beauty many clouds - both noctilucent or otherwise - are the direct result of pollution. More serious damage to the upper atmosphere, and possible implications for climate change, have yet to be established with certainty. It seems very likely that if aviation movements continue at their present rate all of us - not just astronomers - will suffer the consequences.

Nautical twilight lasts all night from Orkney between May 16 and July 28.

JV 15/06