I
Imbolic: an ancient Celtic festival held on the 2st of February, celebrating fertility. It is one of the cross-
quarter days
inclination
[i, I] the angle between the back of the dial plane and the horizontal for inclining or reclining dials. Equivalently, it is the angle between the zenith and the positive z co-ordinate of the dial. i = 0º
implies a horizontal dial. For an inclining dial, 0 < i <
f (the latitude of the place). For a reclining dial leaning away from the observer, i < 90º; whilst i > 90º implies a proclining dial leaning forward towards the observer. Beware: this convention is not followed by all authors.
inclinometer
An instrument for measuring the inclination or slope of a surface. Two types are common: simple devices with a plumb-line hanging across a protractor, or precision ones where a sensitive spirit level is moved to the horizontal position against an accurate scale. Note: the term inclinometer is also used to describe an instrument – also called a dip circle – for measuring the vertical component of the Earth’s magnetic field.
index
Besides its normal meaning of an alphabetical list, an ~ is a pointer on a scientific instrument, indicating a point on a graduated scale. The index arm of a sextant is the movable arm carrying the index mirror and the fiducial line.
inferior
Refers to an event on the celestial sphere below the horizon. Opposite of superior.
inhiraf
The angle which the qibla line makes with the north ray of the meridian at any location.
International Date Line
The line from the N to S poles, approximately following the 180º line of longitude, through which the date alters by one day (positively if travelling from W to E). Variations from the 180º meridian are made to avoid political and geographic boundaries.
Islamic prayer lines
The lines on Islamic dials where the shadow of the nodus falls at the times when Muslims must pray. The times of the three most common lines are determined by a linear relationship to the noon shadow length of a vertical gnomon:
zuhr
Noon shadow + 0.25 x gnomon height
asr-awwal
Noon shadow + gnomon height
asr-tânî
Noon shadow + 2 x gnomon height
isogonals
Lines of equal magnetic deviation plotted on some navigational charts.