# Sundials & Starfinders

## Presentation on theme: "Sundials & Starfinders"— Presentation transcript:

Sundials & Starfinders

Sundials Sundials come in variety of forms but all sundials measure “Solar Time”

Sundials Each hour on a sundial is 15° wide. But……. Basic Parts:
Hours markers Gnomon – A device that cast a shadow on the hour markers. Basic Premise: If we divide the day into 24 hours and the Earth rotates about 360° each day then, Each hour on a sundial is 15° wide. But…….

Sundials The Gnomon must be aligned with the Earth’s rotation axis for the sundial to work properly.

Sundials There is an alternate version of a sundial called an “Analematic Sundial” This style of sundial uses a vertical gnomon. However, the gnomon must be movable and calendar positions must be determined for each day’s gnomon location.

Sundials Analemmatic Sundial

There’s still a problem
Sundials There’s still a problem

Sundials The Solar Day is not always 24 hours long due to our changing orbital speed. The Solar Day can be 15 minutes longer or shorter than the clock day depending on the time of year.

Sundials

Sundials The Result: If we want the sundial to read the same time as the clock, we need to add or subtract a correction factor This factor may also include Daylight Savings Time.

Orient sundial so that the gnomon points north. 2. Read the hour markers to determine solar time. 3. Add or subtract appropriate correction factor.

Sundials Try some examples
On October 13th, what time is it if the sundial reads 2:30 pm? On January 25th, what time is it if the sundial reads 10:00 am? When is there no correction to the sundial? When is Daylight Saving Time the only correction?

Sundials Another use for the Sundial is finding directions.
To find your directions: Use a clock to find the time. Perform a reverse correction to determine the correct sundial reading at that time. Turn sundial until it reads the corrected time. You are now facing north.

Starfinders In ancient times, starfinders were called astrolabes.
They were typically made of brass or wood. Astrolabs rarely had constellation names, stars only.

Starfinders Identifying stars or constellations:
1. Line up the time of night with the current date. 2. Hold the direction your are facing at the bottom. 3. The constellations (or stars) will be directly in front of you .

Starfinders Rising or setting times:
Place the star or constellation on the eastern or western horizon of the starfinder. Read the time for that date.

Starfinders Finding the time:
Rotate starfinder until it matches the sky. Pay special attention to the Dippers and Cassiopeia (looks like a “W”). Read the time next to your current date.

Starfinders Which constellation is just above the eastern horizon at 10:00 pm on the 14th of October? Which constellation is just above the southern horizon at midnight on January 15th?

Starfinders

Starfinders What time does time does Sirrius rise tonight?
What time does the backward questions mark of Leo rise tonight? What time does Aquila set tonight?