Finding North Direction and Time by Any Bright Star.

by tonytran2015 (Melbourne, Australia).

Most people living in the Northern temperate and arctic zones know how to find the star Polaris to find North direction and can even tell time by the orientation of the Little Dipper (part of the Little Bear). But Polaris is not an easily seen bright star and may even be hidden from view for people in Tropical zone and in Southern Hemisphere.

This posting shows how to use ANY arbitrarily known star to find North direction and time, and is useful to non-users of Polaris star. The method is similar to that given previously using the Sun [1] and is useful when only one bright star is visible in an unclear sky (affected by thick clouds or pollution).

The divider compass in this posting is only for instructional purpose and is not needed in actual application. The user may use any two of his fingers instead.

(This Instructable replaces an older one with an unsuitable web name: https://www.instructables.com/id/Test-55/ of April 12, 2016. The problem was caused by a work around on a non-publishable draft.).

Note for sky observers:
While looking for Boote Arcturus you may notice that the Scorpion constellation looks different from its drawings. A rare cosmic event is happening to Delta Scorpii at the current time of April 2016: The star is far brighter than usual. You may find out more details about this rare visible event with a search on the internet.

Step 1: The Date of a Star.

Figure: Table of bright stars with their approximate dates .

The date of a star should be known, even only approximately by the month, if the user wants to tell time from that star.

Any star is visible nightly (either from Sunset to its setting or from its rising to Sunrise) for more than 9 months each year. The visibility cycle for each star begins with the star seen rising near the Eastern horizon few minutes before Sunrise. On subsequent days, the star rises earlier and earlier, it travels gradually towards the West and remains for longer and longer duration in the night sky until one day it stays for the whole night. The star is therefore called a star of that date. After that day, the star is seen setting in the West in the night. On subsequent days, its lead on the Sun gradually increases and it sets on the West at earlier and earlier time. Near the end of the cycle, the star is visible above the Western horizon for only few minutes after Sunset. It then sets on the West. At the end of this cycle, the star is too close to the Sun to be visible in the sky. The cycle then repeats from the beginning.

Example:

Boote Arcturus is a star of April 25th with declination 19 degree N. Step 6 of reference [2] shows how to easily identify it.

It reaches its highest elevation at mid-night of that date and remain visible for that whole night.

In February it rises about 4 hr after Sunset and has not enough time to complete the journey to the West. On April 25th it rises at about Sunset to complete the journey to the West about Sunrise. In June it is seen already high in the sky at Sunset and set on the West 4 hr before Sunrise.

Step 2: Selecting a Star for Current Month in the Year.

Figure 1 &2: Inversion maps for Northern and Southern Celestial hemispheres.

Select a bright star of a date in the year near to your current date (or month) and read its declination from any source such as the internet, your own tables or the two star maps supplied here. If you use the star maps, its declination is read from the constant declination circles and its date from the rim on the opposite side (that is also the point closest to the Sun on the elliptic circle).

This method requires POSITIVE IDENTIFICATION of your chosen star. To positively identify it, you may need to see

1/- either neighbouring bright stars, their relative distances and orientation

2/- or surrounding dimmer stars making up the constellation containing it.

When that star is the only visible one in the sky you may have to identify it relying on its appearance details in previous nights such as:

1/- Continuity of its characteristics from previous nights (or hours),

2/- its elevation after Sunset,

3/- its relative directions from the setting Sun or the (moving) Moon,

4/- its ranking as the few brightest objects in the sky after Sunset.

Stars near to the elliptic may sometimes be confused with much brighter planets and their use involves extra cares.

Step 3: Rules for Turning to Lower Celestial Pole.

Figure: The rule applied to the chosen star is similar to the rule illustrated here for the Sun.

Be certain of your hemisphere and whether the star is rising (early star) or setting (late star) to turn the second leg of the divider to the left or to the right. This is similar to the requirement for finding North using the Sun.

Then apply all the steps of the next section.

Step 4: Steps to Find North Direction and Time Using a Star.

Figures: Summary of steps to find North direction and time by a star.

When any star is used on its date in the year, the Sun leads it by exactly 12 hours. For every month after that, the lead by the Sun is reduced by 2 hours (or every 12 months by 24 hours). The last figure of the illustration shows the Sun leading the star by about 5 hours on the Celestial clock face.

Example 1:

In December, January, February use Sirius if you can see it.

Sirius is a star of Jan 1st with declination 17 degree S. Step 8 of reference [2] shows how to find it.

On Jan 1st the time determined by Sirius is 12 hours behind the time by the Sun.

On April 1st, the time determined by Sirius is 6 (=12-3*2) hours behind the time by the Sun.

Example 2:

In March, April, May use Boote Arcturus if you can see it.

Boote Arcturus is a star of April 25th with declination 19 degree N. Step 6 of reference [2] shows how to identify it.

On April 25th the time determined by Boote Arcturus is 12 hours behind the time by the Sun.

On May 25th, the time determined by Boote Arcturus is 10 (=12-1*2) hours behind the time by the Sun.

On June 25th, the time determined by Boote Arcturus is 8 (=12-2*2) hours behind the time by the Sun.

Step 5: References.

[1]. tonytran2015, Finding North direction and time from the Sun using bare hands, Instructables, https://www.instructables.com/id/Find-North-directi... , posted on 2015 may29.

[2]. tonytran2015, Finding North direction and time by stars, Instructables,

https://www.instructables.com/id/Find-North-directi... , posted on Jul 24, 2015.