In parshas Terumah (Shemos 27:1) the Jewish people received instructions to build the alter.
A. The width of the Courtyard of The Mishkan (The Tabernacle) was 50 amos [see “A”] (Shemos 27:12). The width of The Mishkan is 10 amos [see “B”] (Rashi on Shemos 26:23). The space between the sides of the Mishkan and the sides of the courtyard is the same on each side [see “C”]. The Mishkan was exactly in the middle of the courtyard. (Rashi on Shemos 27:18) How much is the space between the side of the Mishkan and the side of the courtyard?
(A – B) / 2 = C
(50 – 10) / 2 = 20 amos
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B. The Outer Mizbe’ach (The Altar) was 5 amos by 5 amos. [see “D”] (Shemos 27:1). In the center of The Mizbe’ach was The Ma’aracha (The Woodpile). The Ma’aracha was 1 amah (cubit) by 1 amah [see “E”] (Talmud Zevachim 62a) [see “B”]. What is the space between the sides of The Ma’aracha and the sides of The Mizbe’ach [see “F”]?
(D – E) / 2 = F
(5-1) /2 = 2 amos
C. The area in front (i.e. to the east) of The Mishkan is shaded in the picture. At least some part of The Ma’aracha (woodpile) had to be in “front” of The Mishkan [Tosafos on Zevachim 63a). This means that if the south side of The Ma’aracha is lined up with the north side of The Mishkan, then it is a fraction too far north, so it would have to be moved further from the wall of the courtyard. If the Outer Mizbe’ach is placed as close as possible to the North wall then the distance between the south side of the Outer Mizbe’ach and the south side of the Outer Courtyard will be a bit less than what distance (in amahs) [See G]
(C + B) – F = G
(20 + 10) – 2 = 28 amos