Mathematical_Recreations-Kraitchik-2e

The Calendar 113
scale). For the months January and February the Year should
be diminished by 1.
2. With a straight edge, join the point on the first scale
given by the Date with that given by the Month on the third
scale, and mark the point of intersection with the second
scale.
3. Join the point thus found with the point on the fourth
scale given by the Century, and mark the point of intersection
with the third scale. (Use the heading Gregorian or
Julian for the Century, according to whether the date is New
Style or Old Style. The date of adoption of the new calendar
varies in different countries, but was 1582 for most of Europe.)
4. Join the point found in (3) with the point given by the
Year in the first scale. The point of intersection with the
second scale gives the day of the week.
Another arrangement is shown in Figure 18.
For successful use of the calendar in Figure 18, its plan and
terminology need to be well understood. Observe that it
consists of three concentric rings intersected by seven radii.
Each radius is identified by the name of a day of the week.
Where each radius intersects the innermost ring is a set of
numbers; these are Year numbers - 42 for 1842, 1942, and
so on, is at the point where this ring meets the Tuesday radius.
Again, where each radius cuts the intermediate ring, more
numbers are found, prefixed either by a J (for Julian) or a G
(for Gregorian; these are Century numbers, as 19 for 1942,
and Gregorian 19 is found where the Friday radius cuts the
intermediate ring. Where the radii meet the outer ring, the
month names are found inside and the day-of-the-month
numbers (Dates) outside. We have then to consider Dates,
Months, Centuries, and Years in order to find the day of the
week on which a known date fell or is to fall. A ruler and
pencil may be needed at first; after a little practice they can
be laid aside; points mentioned in these instructions are al-