L

Lambert’s circles

: circles of construction used when drawing sets of nested ellipses to represent analemmatic dials for different latitudes, these dials using a common scale for the (vertical) gnomon position. Such sets of dials are particularly useful for a solar compass, e.g. the Cole sun compass used in N. Africa during the Second World War. After the mathematician Lambert (b. 1728, Alsace).

Lammas ( or Lammas Day): one of the cross-quarter days. It is on 1st August, and was formerly observed as the harvest festival.

latitude

[f
, PHI, Lat] Note: avoid LAT, since it implies local apparent time. It is the angular position of a place north or south of the equator. Positive values in the Northern hemisphere, negative in the South (i.e., the South Pole has
f
= -90º). Part of the geographic co-ordinate system, the term comes from the Greek “latus” (breadth).

leap second

An extra second inserted into UTC at the end of some years between 24:00:00 Dec 31 and 00:00:00 Jan 1 to ensure that UTC remains in step with the Earth’s diurnal rotation. It may also be added at the end of June. The addition is not predictable as it depends on many factors, such as the increased atmospheric drag on the Earth in El Niño years. The actual addition is performed by the Bureau International des Poids et Mesures near Paris. Leap seconds are gradually becoming more common as the rate of the Earth’s rotation slows due to energy dissipation by the tides.

leap year

Years in which an extra day (February 29) is introduced so that the (Gregorian) calendar keeps step with the Earth’s orbit. The rule for leap years is that a year is a leap year if and only if the year number: is divisible by 4, except years divisible by 100 which are not leap years unless they are also divisible by 400. This corresponds to the length of the year being 365.2425 mean solar days. This can be compared to the 365.25 days in the earlier Julian calendar. (The Julian leap year doubled February 24.) The leap year system causes the EoT (and Sun’s declination) on a particular day of the year to exhibit a small periodic variation.

lemniscate

The term used in Latin countries for the analemma. From lemniscus, meaning ribbon. In English, the ~ is a mathematical curve which is similar to a spiral and is sometimes used in road design; it also looks similar to one lobe of the analemma.

libration (of the Moon)

The periodic oscillation of the Moon from ‘side to side’ (and ‘up and down’) which allows an observer on the Earth to see somewhat more than half its surface.

limation

Had the meaning ‘filing or polishing’.

limb

Part of an astrolabe, it is the circular ring with a scale of hours and degrees. Its first recorded English use was in 1593.

limb

. The term limb darkening indicates that the disk of the Sun does not have uniform brightness but is dimmer around the “edges” due to increased optical absorbtion by the photosphere.

local apparent time

[L.A.T.] solar time see LAT (types of). Hence local apparent noon, at the Sun’s superior transit.

local hour angle

. Beware, this convention is not universal.

lodestone

A naturally occurring oxide of iron, mounted with two iron poles in a non-magnetic frame. Used for magnetising compass needles, small ones were made specially for portable dials.

longest day: a term in common parlance, defined as the day of the year with the greatest (astronomical) sunrise to sunset period. It is normally used synonymously with the summer solstice although, strictly, it can vary by a day depending on the exact time of the solstice and the relationship between the rate of change of the EoT and that of the local sunset/sunrise.

longitude

[?
, ?_t, LON] the angular location of a place on the Earth’s surface measured east or west of the Prime meridian though Greenwich. Longitudes W are positive, E are negative. Part of the geographic co-ordinate system, the term comes from the Greek “longus” (length). See also Prime Meridian.

Longitude Act

A 1715 act of the British parliament which established a Board of Longitude to manage a prize of £20,000 for a practical method of finding longitude at sea.

longitude correction

The correction required to local apparent time to translate it to the L.A.T. for the central meridian of that time zone. The correction is +4.0 minutes for every 1º longitude W of the time zone meridian (and –4.0 minutes for E). Sometimes, this correction is built into the hour lines by calculating the local hour angle for times at the zone meridian.

lunar angle

The difference between the right ascensions of the Sun and the Moon. On a standard sundial used as a moon dial, the L.A.T. equals the time shown by the lunar shadow plus the lunar angle expressed in hours.

lunation

The time interval between successive New Moons. The mean interval is 29 days 12 hours 44 minutes 3 seconds (the synodic month) but, because of the perturbing action of the sun, the difference between the shortest and longest lunations in the 20th century is 5 hours 19 minutes.