How to Determine Directions Using a Wristwatch

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1. Introduction

⑴ When the hour hand points to the sun, the direction that bisects the hour hand and 12 o’clock is south

① As seen in Scorpion Season 1 and Detective Conan.

⑵ Related to the diurnal motion of the celestial sphere

⑶ Example 1. Sunrise : Assuming 6 AM, with the hour hand pointing east, the 9 o’clock direction is south

⑷ Example 2. Noon : 12 PM, with the hour hand pointing south, the 12 o’clock direction is south (cf. solar noon altitude)

⑸ Example 3. Sunset : Assuming 6 PM, with the hour hand pointing west, the 3 o’clock direction is south

⑴ Simplification : Sunrise at exactly 6:00 AM in the east, sunset at exactly 6:00 PM in the west, the solar path is parallel to the horizontal plane

① Part of the solar path is above the horizontal plane, and part is below, hence the concept of day and night

② The angle between the solar path and the celestial horizontal plane is 90 - φ° (where φ : latitude)

⑵ Diagram

Figure. 1. Proof Diagram

① The sun is t° (-90 ≤ t ≤ 90) from the south

② ⇔ The current time T is T(t) = 12 + t / 15 (hours)

○ Reason 1. (t, T) = (-90, 6), (0, 12), (90, 18) are passed

○ Reason 2. T = T(t) forms a straight line graph

③ ⇔ The angle between the hour hand and the 12 o’clock direction is 2t°

○ Reason 1. For every hour past 12, the angle of the hour hand widens by 30°

○ Reason 2. 30 × t / 15 = 2t

④ ⇔ The south direction is the direction that bisects the hour hand and 12 o’clock

⑴ Using an analog watch

⑵ Only possible at noon

① South : Direction of the sun

② North : Direction of the shadow

③ East : When facing away from the sun and looking at your shadow, the right direction

④ West : When facing away from the sun and looking at your shadow, the left direction

⑶ Finding directions at night

① Direction of the North Star

② By coincidence, the North Star is currently in the direction of Earth’s axis of rotation

③ Due to precession, the North Star may not always be in the direction of the axis

Posted : 2019.07.31 20:14

Updated : 2024.03.11 00:22