Sky Light Now App - Global Sun and Moon Illumination

Written and Published by Lawrence Freytag

Latest Website Update May 23, 2022

 

DISPLAY of Sky Light Now (landscape mode) for a particular latitude, longitude, date and time:

 

App Operation Explanation:

 

The app displays the day's morning and evening twilight stages, as well as the Sun and Moon rise, transit, and set times. The graph shows Sun elevations in yellow and Moon elevations in blue. The positive elevations show when they are visible, the negative elevations show when they are below the horizon and invisible. The white ticks show the progression through the twilight stages, these are also listed by name and time. Each stage corresponds to 6 degrees of elevation below the horizon. This enables understanding what light will be available throughout the day from the Sun and Moon. The prediction is based purely on astronomical factors, the weather is not taken into account.

The user-entered location name and geographic coordinates are shown at the heading of the display. Manual or GPS entry of location are both available.

Based on the date and UTC time input from the cell phone, and the latitude and longitude entered, the correct geographic time zone is selected and the display is calculated. These are called military or nautical time zones. They are based purely on longitude. Each zone is 15° or 1 hour wide. Due to the need to work worldwide, civil time zones are not used. At noon, the Sun will pass exactly over the observer's longitude if they are in the geographic center of the zone. These zones are listed below.

Below the header information are two NOW lines of information. These tell you the status of the Sun and Moon at the moment the display was written. This shows the Sun being up or dark, and twilight phase, and how high is the elevation. The Moon line indicates current elevation and also the percent illumination (new moon is 0%, full moon is 100%). The time corresponds to the red vertical dashed line on the plot (normally would be called the cursor). The Sun is either up, or down, but twilight occurs before sunrise and after sunset.

Following is a nine-entry table of Sun events and times. The twilight stages are indicated, along with daylight and dark periods. The moment and elevation of the day's highest Sun elevation is also indicated in its own line. This is often called the Transit time. The Moon illumination is not considered in this table.

On the right hand side of the display is a fairly busy graph. The yellow trace shows the Sun elevation. The purple trace, the Moon elevation. If these are above the zero line, the body is visible to the observer, otherwise below the horizon. The white tick marks on the Sun trace detail the twilight stage times. The graphs proceed along a 24 hour time X axis.

Finally, the classical Sun and Moon times are indicated in panels just above and below the graph. These are in time alignment with the graph. So for example, if you look upwards from the Moon Set mark, you will see the purple graph line crossing the zero elevation point moving downwards. The elevation graphs provide a more complete expression of expected illumination than the times by themselves

The last item under the graph in the Moon panel indicates the Moon phase and percent illumination. Full moon is 100%, new Moon is 0 %, the quarters will be 50%..

 

User Controls

The Location icon is used to call up the Location Entry display. In this you give a name you want the location known as, and the latitude and longitude which can be entered in decimal degrees, or in degrees/minutes/seconds. The example below shows the coordinates entered as decimal degrees. You can just enter the numbers for each then select the N/S or E/W buttons If you are entering coordinates in Degrees, Minutes, Seconds just type them in with spaces in between. You only have to input the figures you have, you can put in degrees, minutes, and seconds, or degrees and minutes, or just degrees. You can use also your phone's GPS to input the latitude and longitude of your present location.

When you enter the location, the display is calculated and shown immediately and that location will be remembered by the app. There are eight storage locations provided by the shortcuts, so if you will be reusing the location save a shortcut. The ... commands at the end of the header bar include a save shortcut command.

The refresh icon just updates the display to the current time, so if the display has been setting there for awhile and you hit refresh the time cursor will move to the right and the NOW line will be updated.

The calender icon accesses the calender, you can set any date you want to see the display for. By setting the date to December 21, for example, you will see the plots move downwards, illustrating the longer nights in winter (north of the equator). Going to June 21 moves the plots upwards dramatically especially in mid latitudes and above. Here, it is clear why it never seems to get really dark at night, twilight is about all you will get in summer especially up north. Hitting the refresh button returns the display to the present date.

Inside the ... commands, are eight individual commands you can use.

The first is Add Station to Favorites, already described.

The next is GMap Station (internet). This will pass the coordinates you have entered to the Google Maps app on the phone, and along the way you can choose from various zoom factors of 1, 10, 50, 100, or 500 miles display width. As indicated, Google Maps will use the internet to gets its map, that is the only way you can get this app to access the internet. All the formulas are built into the program. Just use the back key to get back to the app from GMAP.

The next command is Display Bright/Dark. If you are out at night it will tone down the bright display, if you are out in the Sun you can make it easier to read.

After that is somewhat misnamed Sun and Moon Info. You can use it to remove the Sun and Moon info panels to get a larger graph.

Next is Time Format, you can use AmPm or 24 hour time. 24 hour time works best in extreme latitudes, because the events come closer together in those locations, and the time is displayed more briefly.

Next is Manual DST Setting. Even though the nautical/military zones do not support Daylight Savings Time, I have found that it is very helpful when the displayed times match the local time (i.e. the time on your watch). You can turn on the DST Shift to accomplish this. You have to do this manually at the start and end of the DST period.

Next is About Gestures, it reminds you of the 3 gestures available. Swipe to next day, prior day, or access shortcuts.

Next is Grid Control, there are three levels of grid available to clean up a busy display.

Finally, there is Contact Info, it gives you my email address and I urge you to shoot me a note or question, these are very helpful to me. I will always get back to you, if you are having some problem with the app please let me know and we can work on it over email. If you have a suggestion also please let me know it really helps me plan future work.

Find the application here

May 18th, 2022 --- Sky Light Now Version 3 released

This is the third release of my sky illumination app. It has been updated for Android 12 and a GPS utility has been added
to read in your present location into the app.