Finding true North and time from the Sun with your fingers.
by tonytran2015 (Melbourne, Australia).
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(Blog No. 149)
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Finding true North and time from the Sun with your fingers.
There are times when you neither have your watch nor can use any magnetic compass in the location but you want to find out the North-South directions and the time. This method is useful for those such difficult situations. Those situations may arise if you get lost without having your watch while traveling or if you find yourself without your watch while traveling inside a bus or a train. The method from this article gives both the true North direction and the local time from the position of the Sun using only your fingers.
Required preparatory practices
1. Practice holding each of your hands in the three principal postures as illustrated in the following three figures.
Figures: Hand postures for Summer Solstice, Equinox and Winter Solstice. Click on individual figure to enlarge.
If this practice cannot be carried out due to body deformity or illness (such as rheumatism) then some other method of finding North should be used instead.
The equinox posture is to be used around Mar 21st and Sep 23rd equinoxes while Summer and Winter Solstice postures are to be used around your local Summer and Winter solstices respectively.
The index finger in these postures is always aligned with the forearm and is to be kept in line with the line from the elbow to the tip of the index finger.
The angle between the index and the middle fingers should have value of:
90 degrees for Equinox posture
90-23= 67degrees for Winter Solstice posture and
90+23= 113 degrees for Summer Solstice posture. The above angles for Summer Solstice, Equinox and Winter Solstice postures are equal to the angles between a clock hand pointing at 0 minutes and
19 minutes,
15 minutes,
11 minutes respectively. These angles are represented by angles between positions of watch hands on a watch face shown in two following figures.
Figures: Angles between fingers for hand postures at Summer Solstice, Equinox and Winter Solstice are represented by angles between positions of watch hands on this watch face. The long white hand pointing at 0 minute of the watch-face represents the direction of the left index finger of the user of this method while the long red hand represents the direction of his left middle finger (see text).
The long white hand pointing at 0 minute of the watch-face represents the direction of the left index finger of the user of this method while the long red hand represents the direction of his left middle finger.
The angle between the hands on each watch-face has been chosen to match the angle between the line to the Sun on the respective date and the line to the Celestial pole below the horizon. The angle between the red hand and the thick white hand pointing at 15 minutes represent the declination angle of the Sun (or its negative, depending on the observer being in the Southern or Northern terrestrial hemisphere). The variation of that angle through various dates of the year can be found in previous blogs [1,2].
Figure: Determining solar declination using a watch face. (The lines “SOLAR DECLINATION Its rough estimate is required for Fine Alignment of the watch” are to be ignored.)
2. Determine the slope to your Celestial pole.
Figure: The Sun, the Moon and the stars are attached to a Celestial sphere which encloses the Earth like a giant rotating cage. The cage rotates around the Celestial axis (in cyan-blue color) joining the its two points called the Celestial poles. The horizontal ground of an observer at the center of the celestial sphere is represented by the horizontal great circle of the Celestial sphere while his line of sight to the Celestial pole is represented by the cyan-blue arrow.
The slope from level ground surface to the line of sight to the visible Celestial pole is called the latitude of your place. Practice recognizing it.
Find a level ground. On a clear night set up a stick pointing from the ground to the Celestial pole. In the Northern hemisphere the Celestial pole has a star (Polaris) while in the Southern hemisphere it is only a point on the geometrical figure formed by circum-polar stars. Such a stick is constructed as a shadow rod in any “builder clock”.
Figure: A “builder clock”. The shadow rod of this clock is set to point towards the Celestial pole in the sky.
The inclined shaddow rod on a “Builder Clock” points toward the Celestial pole in the sky when the clock is properly setup with its base in the true North-South direction.
The angle between the stick pointing to the Celestial pole and the ground is called the latitude of the location. The angle between the stick and a vertical plumb line is (90° – latitude). You need to practice recognizing this angle. (Knowing this angle also help you quickly find the Celestial pole from the stars).
Figure: The Northern Celestial pole is the center of this map of the Northern sky.
Figure: The Southern Celestial pole is the center of this map of the Southern sky.
3. Practice reading in degrees the angles between positions of hands on a clock face.
The angle between a hand pointing at 0 minute and another one pointing at
5 minutes is 30 degrees,
10 minutes is 60 degrees,
15 minutes is 90 degrees,
20 minutes is 120 degrees,
25 minutes is 150 degrees,
30 minutes is 180 degrees.
8 Steps for finding true North and time.
1. Determine the current season in the year to select the appropriate hand posture.
The equinox posture is to be used around Mar 21st and Sep 23rd equinoxes while Summer and Winter Solstice postures are to be used around your local Summer and Winter soltices respectively (Each posture can be used for its whole month and a solstice posture can also be used for two adjacent months.).
If the season in the year cannot be determined (as in the case of inhabitants living in artificial environment for years), use the hand posture for equinox days.
2. Determine whether you are in the Northern or Southern hemisphere.
3. Determine if you are in the morning (the Sun is rising before noon) or in the afternoon (the Sun is setting after noon)
This step is needed to select the appropriate hand for the task.
4. Select and use only the appropriate hand for the task:
4a. Northern hemisphere: LEFT hand in the morning THEN RIGHT hand in the afternoon.
4b.Southern hemisphere:RIGHT hand in the morning THEN LEFT hand in the afternoon.
5. Point the index finger to the Sun with your middle finger in its comfortable, nearly horizontal position.
6. Twist the forearm and hand until the middle finger makes with the level ground an angle equal to the latitude angle.
This is illustrated in the two figures.
Figure: Finding the meridian (true North-South) line with the left hand.
Figure: Finding the meridian (true North-South) line with the right hand.
7. The projection of the middle finger onto the ground now points exactly away from the terrestrial pole of your hemisphere.
The middle finger now points to the Celestial pole below the horizon, in other terms it points directly away from the visible Celestial pole in the sky.
8. Looking along that direction pointed by the middle finger and imagining a 24-hour clock dial attached to that axis give a natural clock giving time in the day.
Figure: The line CB to the Sun form the hour hand of a 24 hour clock. This clock face is for Northern hemisphere. In Northern hemisphere the hand sweeps clockwise while in Southern hemisphere it sweeps anticlockwise.
The time given by the natural clock is the local time which has noon when the Sun is highest in the sky. Local time differs from the zonal time selected by the government.
9. Around noon time, either left or right hand can be used. The terrestrial North South line is determined with least accuracy around noon time.
10. On the terrestrial equator, either selection of 4a or 4b can be applied. The middle finger of the selection 4a points at true terrestrial South while that of 4b points at true terrestrial North.
Figure: Summary of the method of finding true North and time from the Sun.
References.
[1]. tonytran2015, Finding directions and time using the Sun and a divider., posted on May 6, 2015.
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