The Earth-Sun Relationship

Description

The aim of this applet is purely educational. At first glance it may look pretty ordinary and uninspiring - however, once you start interactively dragging around the location marker and overlaying information such as time zones and sun paths, a lot of the subtle detail suddenly becomes blindingly obvious. Different users will likely discover different stuff, so there is no easy way of summarising everything that is there to be found other than by simply playing around with it.

How to add overlays and change options is described in the sections below. However, the following are some things to know and try when you first start out:

• Turn on the Antarctic/Arctic Circle lines and move the view so that you can clearly see the North Pole. Start the Hourly Solar Cycle animation (F8 or Ctrl+Q) and, while it’s running, drag around the Date slider at the top of the applet. You should see that there are times of the year when the Sun never falls within the white circle, and at other time it never stops falling there.

• As you move around, you should notice a small red sphere and cross-hairs on the Earth’s surface. This represents a specific location at which the details in the Solar Information Panel are taken. You should be able to click and drag this location around. If you display the Annual Sun Path and the World Time Zones, you will see the position of the Sun and the Sun path jump each time you cross a time zone. This is because the time slider value stays the same but the reference longitiude changes, resulting in a jump in the difference between Local Time and Solar Time.

• Turn on the Annual Sun Path, choose to view from the Current Sun Position setting and then start the Annual Solar Cycle animation (F9 or Ctrl+A). You should now see an small yellow sphere indicating the point on the Earth’s surface that is directly beneath the Sun. You will notice that this point travels in a figure-8 pattern called an Analemma.

Visualisation Options

Use the EARTH button (F3) to display the main options menu. This allows you to change the way the Earth is viewed, choose what data is overlaid, select important dates and start/stop then animations.

VIEW EARTH FROM:

The default view is from Any Angle. This allow you to rotate around and view the Earth from any direction. If you choose Geostationary Orbit, the view will be locked to directly above the location marker. As you rotate the view or drag the marker, the location and view will change to remain locked this way. Alternatively, if you choose to view from the Current Sun Position, the view will change to an orthographic view (as the Sun’s rays are effectively parallel when the pass the Earth so it does not ‘see’ any perspective distortion) and only changing the date or time will make a difference.

OVERLAY INFORMATION:

These items toggle the information that is overlaid on the Earth’s surface.

SET DATE/TIME:

These items set the date and time to the different seasonal Soltices and Equinoxes. The seasons depend on the hemisphere that the location marker is currently in, so they will be different in the northern and southern hemispheres.

ANIMATION:

The animate either the time slider or the date slider. This means that the annual cycle is performed for the currently selected time. Even when animating you can still interact with the applet as you would normally. The Stop Animation item becomes active only when actually animating.

General Options

You can use the OPTIONS button menu (F4) to toggle the overlay panels that show solar information, the world map and simple instructions. You can also use it to set change the size of the world map or overlay world time zones. You can also select different views of the model.

Again, the only real way to understand the use of these applets is to experiment, so please feel free to simply play around…