Shadow Length Calculator

Mobile App Specification
Version 1.2

1. Introduction
The goal is to develop and implement a mobile App that can help people carry out simple experiments around the world relating to the Sun's shadow length. However, more important than providing a calculated shadow length that people can they verify for themselves with their own empirical experiments, the App will demonstrate for people from school children to adults how Earth's rotation, declination and orbit around the Sun on any given day and time of the year will determine the Sun's shadow length. To achieve these aims, this draft document will provide the starting point for a specification for developing the required mobile App.

2. 'How To'
The App needs to give a simple A, B, C type explanation of how to do the experiment and what resources they will need.
note: As written, the information an steps described are not 'user friendly', especially for young school children. There will be more dynamic and interesting ways to display this information such as with images and so on.
eg.
What you need to do this experiment with this App:
a) A day with enough sunshine to cast a clear shadow
b) A straight stick or rigid wood/plastic rule, ideally 1 meter long
c) Optionally, some stones, bricks or similar to put at the base of the stick/rule to hold it vertically upright
d) A tape measure (metric)

How to do the experiment:
a) Find a nice level area of ground without shadows from nearby buildings, trees, etc
b) Locate the stick/rule standing vertically upright on the ground - better if you put some bricks at the base to hold it vertically upright
c) If you have a carpenter's level, you can adjust the stick/rule so it is perfectly upright but, if not, just estimate with your eyes if the stick/rule looks to be vertically upright.
d) Be careful to measure the height of the stick/rule from the ground to the top of the stick/rule
e) Enter the height of the stick/rule into this App
f) Check the calculated length of the shadow that the App displays
g) Use your tape measure to measure the shadow and compare if the measurement is the same or near enough to the calculated shadow length
h) Repeat at different times of the day and days of the year
i) Ask your friends or family who live in different parts of the country or the world to download the App and repeat the experiment!
j) Learn more about WHY the Sun's shadow length changes the way it does depending on your location, day and time of the year by following the links provided.


3. Inputs
3.1 Latitude and Longitude
This should ultimately be provided by the Mobile Phone's location finding capability.
Initially, for a staged development approach, the Latitude and Longitude can be manually entered.
In a later stage of development, the default values can be automatically calculated by current location but should still be capable of being overwritten by the user if they choose to enter different valid coordinates.

UI considerations: The user should be informed that they can override their location's Latitude and Longitude if they wish.

3.2 Local Date & Time
By default, the current local time can be automatically entered but can be overwritten by the user if they wish.

Note: The application will have to allow for DST (Daylight Saving Time)

3.3 Height of object casting a shadow
For the experiment, the user can input the length of stick or whatever object they wish to use to stand vertically on level ground to measure the cast shadow.
Units can be fixed to meters and decimal parts to 2 decimal places.
By default, the value of 1.00 can be entered.

UI considerations: A UI design feature can incorporate information/guidance/tips for the user such as, advising a very level surface use used for the experiment and suggest that a stick or wooden rule of known length is used such as a 1 meter wooden or hard plastic rule. Suggestions can be made about holding the stick/rule in place using bricks at the base so they don't have to physically hold the stick/rule while measuring the shadow.  They can also use a carpenter's level to ensure the stick/rule is accurately vertical.

4. Calculated variables
4.1 UTC Date & Time
Can not be changed by the user but will be updated if the Local Time is changed by the user (ref. 1.2)
nb. It is assumed that calculations required for the Sun's shadow length should use UTC time as local time could have DST (Daylight Saving Time) adjustments.

4.2 Other constants & variables according to the shadow length calculation
Further work required to complete this section but will involve known and readily available calculations so this should not be difficult.

5. Outputs
5.1 Solar Noon using Local Time (DST adjusted if necessary)
Although this is not related to the local time input at 1.2, it's useful to show when solar noon occurs at the given latitude and longitude on the same day of year relating to 1.2.

5.2 Shadow Length at Solar Noon
Units: Meters to two decimal places
Can Label this output as something like, 'Shadow Length at Solar Noon (Local Time: hh:mm:ss):'

5.3 Shadow Length at current Local Time
Units: Meters to two decimal places
Label: 'Shadow Length at current local time, hh:mm:ss:'

Note: As the application will use UTC time and not DST time where it applies, the output current local time needs to be converted back from UTC to DST time where and when applicable.

5.4 Angle of Sun (Degrees)
The shadow length will of course be relative to the height of the stick/rule and the angle of the Sun. Therefore, we calculate the angle of the Sun that is necessary to create the shadow of a certain length, the angle will be same even if we used different length sticks/rules.  Because the vertical height of sticks/rules or other suitable items will vary, this will make it easier to compare different results to experiments carried out in the future or experiments done by other people in other locations.

5.5 Graphical displays
This is a future development stage so just needs to be allowed for at this point and not implemented.
It is hoped that a 2D image can be generated that shows a segment of Earth that includes the App user's location at one point and the point on the same line of longitude where the Sun is directly overhead at the given solar (?) time.  A depiction of the Sun's parallel rays hitting the surface of the curved segment of Earth can be shown in relation to the two locations with angles displayed as well as depictions of the shadow length with measurements.

6. Recommend to other people
As part of the App's appeal, it is more interesting to also ask friends and family who live in other locations/countries to download the App and try the experiment themselves.

7. Feedback & Donations
Allow Apps users to send their feedback.
Provide donations facility such as https://www.buymeacoffee.com/ to provide funds for future App enhancements or other similar Apps.

8. Pin Map
Optional and future stage development for the App to post data to a central database each time the App is used.
Data would include date and time, latitude, longitude, stick/rule/pole height, Sun angle.