Stargazing: So what is darkness anyway?

As the Sun sets at the end of the day there is a period of dim light known as “twilight”. This si caused by the rays of the Sun being refracted, or bent, through the atmosphere so that some light still falls on the darkening hemisphere. This state precedes Dawn and follows Dusk throughout the year and depends upon many factors, including the “elevation angle” of the Sun ( whether it drops steeply or in a longer, shallower descent), and atmospheric conditions.

In fact there are three forms of twilight. The first of these is “Civil Twilight, when the Sun is between 0 and 6 degrees below the horizon. It is defined as the period when the human eye finds it difficult to distinguish terrestrial objects at the horizon and the brightest stars and planets begin to appear in the sky. Civil Twilight statutes usually refer to a fixed period 20-30 minutes after sunset rather than the elevation angle of the Sun, and correspond to what is known in the UK as “lighting-up time” – street lights come on and car headlights must be lit. More widely thus is also the time when hunting activities must cease and when “burglary” becomes “nighttime burglary” which carries a heavier penalty in many jurisdictions.

Immediately following Civil Twilight is “Nautical Twilight” which occurs when the Sun lies between 6 and 12 degrees below the horizon. This period is defined as the period when sailors are unable to navigate because they cannot clearly define the horizon, although they may be able to discern sources of light on the horizon.

This is then followed by “Astronomical Twilight” when the Sun lies between 12 and 18 degrees below the horizon. This period is defined as when it is possible to make astronomical observations of point sources of light, such as stars, planets and star clusters, with the faintest naked-eye stars at magnitude 6 only becoming visible at the end of Astronomical Twilight.

For astronomers it is only possible to observe fainter objects such as nebulae and distant galaxies after the Sun has fallen further than 18 degrees below the horizon (Astronomical Dusk) which is when true nighttime begins.

Skynotes

Please note all times given in this article are in GMT so remember to add an hour to get the time in BST.

Mercury: The smallest planet is too close to the Sun to be seen this month.

Venus:  rises just before dawn and can be seen on the western horizon. It rises earlier each night so the best views will be towards the end of the month.

Mars:  Mars is visible, low in the South-West after sunset. It is getting lower and further West each night so the best views will be shortly after sunset at the beginning of the month. The close approach of the red planet to M44, The Beehive Cluster, on May 5th would make a good photo opportunity.

Jupiter:  The giant planet is also rising earlier and lower each night. It is still visible if you have a clear Southern horizon.

Saturn: The ringed planet can also be found rising in the East just before dawn and close to Venus. By the end of the month it will be higher in the sky and easier to see.

Uranus and Neptune: These planets are not visible this month. Uranus lies too close to the Sun whilst Neptune rises close to Saturn in the pre-dawn sky.

Meteor Shower: The Eta Aquariid shower peaks on the 6th of May but, at this time of year, the radiant point sits on the Eastern horizon and reaches its highest point in the sky during daylight. Radio astronomers can track these meteors which are interesting because they are very fast moving (over 40 miles per second) and are associated with Comet Halley.

Noctilucent Clouds:  It may seem anachronistic for astronomers to be interested in clouds, but these rare apparitions are best seen at this time of year.  Noctilucent clouds are so high (over 80 km) in the atmosphere that they are able to reflect sunlight even when the Sun is well below the horizon.  The tiny ice crystals that make up the clouds reflect bluish-white light that stands out against the gloaming sky.  They are typically seen 1 ½ to 2 hours after sunset low in the Northwest or before sunrise low in the Northeast.

The First Quarter Moon occurs on May 4th with Full Moon on May 12th.  Last Quarter follows on 20th  and New Moon at the end of the month on June 27th.  

Data supplied by Dale Giles, Observations Secretary Torbay Astronomical Society 

Diary Dates

The next meetings of the Torbay Astronomical Society will be held at Torquay Boys Grammar School, Shiphay Manor Drive, TQ2 7EL.  On May 8th. This will be our AGM and will be held in in Room PL4 where we will be previewing the exciting speakers and events we have lined up for our 70th Anniversary season beginning in September 2025. For details of forthcoming and future meetings contact the Secretary TAS on astrosecretary@gmail.com .  Visitors and prospective members especially welcome.