by Milan B.
All non-circumpolar stars have seasons of visibility. They depend on how far the star is from the Equator (the declination), but also how far the star is from the ecliptic. Sirius, being a Southern Hemisphere star and lying around 17 degrees below the equator also lies quite a bit below the ecliptic. This celestial geometry results in Sirius’ visibility season stretching roughly between September 1st and May 1st (the following year) for observers near 49 degrees North.
So, we have a few days left for this year’s season and to many it may be obvious how fast Sirius is disappearing in the SW sky twilight these days. All non-circumpolar stars are rising and setting just under four minutes earlier each day, throughout the year, but is this season finale unfolding faster for Sirius than other stars?
The end of season for Sirius coincides with another celestial phenomenon, this one by our own star – the Sun. Around this time of the year the days are lengthening at fast pace as we just past the spring Equinox in March. The sunsets are falling about a minute and a half later each day in April as the Sun is climbing higher in the Northern Hemisphere heading towards the June Solstice. The table below shows how the difference between the time Sirius sets and the sunset shrinks dramatically in the month of April.
|Date||Sirius Setting at||Sunset||Difference||Altitude of Sirius at|
|April 1||23:57||19:45||4h 12min||23.7o|
|April 11||23:17||20:00||3h 17min||21.3o|
|April 21||22:38||20:15||2h 23min||17.2o|
|May 1||21:58||20:28||1h 30min||11.6o|
How dramatic the change is between April 1st and May 1st could be seen in the images taken from Sky Safari. Both images show the SW-W sky at the time of civil dusk, when the Sun is about 6 degrees below the horizon and when the brighter stars become visible.
On April 1st the altitude of Sirius at Civil dusk is 22.4o
On May 1st the altitude of Sirius at Civil dusk is only 6.1o
In normal times I would say: go out and enjoy the last few days of Sirius visibility in the evening SW sky, but we are in different times. Hopefully, next year in April, we will be able to do this without breaking any rules.