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Monroe Calculating Machine (G Model)


The main part of this object (2019.ast.111.1) is a black metal box with a number of keys on the front, arranged in an ascending formation, with a row of short 0 keys at the front and a row of tall 9 keys at the back, with the other numerals in between. A single red key is set to the right to this, alongside a metal knob with an arrow on the top. On the right side of the box there is a rotating crank handle. On the front side of the box there is a key-type knob which when rotated lifts the display carriage and moves it to the left or right by one digit.

Behind the keys, mounted on the back edge of the top of the box, there is a wider section, the carriage, containing the display of the calculating machine. This is has two rows oval holes through which numerals are visible. On the right hand side of the carriage there is a rotating crank handle (the reset knob), and a knob (to lift the display in order to reset the displays). Along side the numerical displays there are metal indicators which can be slid along a numbered rail to indicate specific numerals.

There is a Instruction Book with this object (2019.ast.111.2). This contains instructions on the use and care of the machine.

Accession Number: 2019.ast.111.1-2 (DAA-0044)

Alternative Name: Mechanical Calculator

Primary Materials: Metal: Iron Alloy, Plastic


Painted on the front of the box: “MONROE
Calculating Machine Company
New York, U.S.A.

Painted on the back of the box: “MONROE

Dimensions (cm):

Length = 44, Width = 33.4, Height = 16.5


To aid simple calculation, including addition, subtraction, multiple and division.


Good: The calculator (2019.ast.111.1) shows many signs of use. The keys, crank handles and other surfaces frequently touched during use are worn and dirty. The painted-on label is worn in places. The metal surface of the box and display carriage is rusty in placaes, particularly around the corners.

The booklet is in very good condition. It is slightly worn on the cover and the paper is yellowed, but is in otherwise undamaged.

Associated Instruments:


Monroe Calculating Machine Company, Orange, New Jersey

Date of Manufacture: 1919-1921


This object was moved from the David Dunlap Observatory in Richmond Hill in 2009, upon the sale of the observatory. They were stored at the Department of Astronomy and Astrophysics until 2017, when it was moved to a new storage location in McLennan Physical Laboratories.

Additional Information and References:

Historical Notes:

Although she didn’t use this machine, Department of Astronomy professor Christine Clement recalls using a similar calculator, a Marchant electromechanical calculator, as part of her early research work in determining the periods of variable stars, in the 1960s.
As she remembered in an interview:
“[So] then when you think you have [derived] the period [of a variable star], you get out that Monroe calculator.  Mind you, the machine I had was newer than that, but it had keys that go clickety clack.  Then what I had to do was to assume this was the true period, and test it by calculating the phase for all the different observations I had.  This calculation involved dividing each time of observation (expressed as a Julian date) by the period.  I could store the period, but every time I tested a different period. I had to retype in all those dates of the observations.  Nowadays this can be done at my computer in an instant; it can rapidly try out different periods. [But in 1963] I had to try them out bit by bit by hand, and then I had to plot the light curves on graph paper and might say, “Oops, that one doesn’t work, let’s move on to another period.”…I had to spend too much time using the calculator, plotting graphs by hand, trying different periods, entering all those dates again. Now I would just type them once, if at all, because nowadays, they might come directly from the telescope, along with the rest of the data.  And the thing is that now people can get right to the astronomy. We had to spend so much time… 
There was another problem I encountered with the Marchant calculator … It had keys kind of like an old-fashioned typewriter, you remember the old-fashioned typewriters keys? Well, that’s what this machine had, and sometimes the keys would jam, and then you’re out of luck!  When the calculator keys jammed, we had to call the repair man, and we might have to wait a week before he could come!”            
Clement notes that the mechanical and fault-prone nature of these machines determined how data was presented. Referring to a chart in a published paper, she states:
“I have a column where I list the period [of the variable star]. But then I have another column where I list the reciprocal period. And the reason is that those machines really choke if you try to divide. Division is complicated. It makes it easier for the keys to jam! So everyone—and this was quite standard— published the reciprocal period, because that’s what we really used to calculate the phases.”
Publishing the reciprocal of the period of the variable star meant that calculations using the published numbers could be done as multiplication rather than as division, as division was more complex, and caused more jams in the mechanical components of the calculator.
Christine Clement was interviewed on February 28, 2021. Thanks to Professor Clement for her assistance with this text.