by Milan B
We are approaching the 2021 Vernal Equinox and are witnessing how fast the daylight hours are increasing for observers at 49 N. This phenomenon will affect the visibility of certain stars more than others. Sirius, the brightest star in the sky will be greatly affected by this day lengthening and will go quickly into its summer sleep around mid-May and stay out of sight for about three months for observers at 49N. On the other hand, the mighty red giant Antares is just “warming up” for its’ “opposition” with the Sun in late May – early June, when it will dominate the low southern skies.
So, which of these two famous stars has better visibility for observers at mid-northern latitudes like 45 N or 49 N?
Many would say that Sirius, being almost 10 degrees higher in the sky for northern hemisphere observers is the absolute favourite. But hold on a sec, Antares is not throwing the towel in yet.
The chart below shows the hours of visibility for Sirius and Antares for each Friday in 2021. On the first sight the blue bars dominate the red ones, especially in the months when Sirius is visible for almost 10 hours each night.
But, on a closer look, the gap of invisibility for Sirius seems much wider than the one for Antares, revealing the fact that Sirius is invisible for much longer than Antares.
So, to answer the above question we need to refine the definition of “better” visibility. If we add up all visibility hours throughout the year, we can see that Sirius’ total hours dominate. This is confirmed by the average (for the year) line for Sirius, which is close to 5 hours per day compared to the average line for Antares, which is at around 4 hours per day. However, if we add up all days when each star is visible, then Antares becomes an unexpected winner. It is out of sight for observers at 49N only for about three weeks in late November – early December when the late autumn sun slides just above it, on its steady stroll along the ecliptic.
It is important to mention that for the reason of simplicity, the visibility hours in the above chart have been calculated when the star is above the horizon while the Sun is below the horizon. To compensate for the fact that stars are not visible immediately after rising or before setting, especially if the Sun is not far below the horizon, a one hour correction line was added to the chart. This line will “bite” a lot more into the visibility of Antares, as the mighty red giant spends more time in very low altitudes of just a few degrees above the horizon compared to Sirius.
Even if we subtract three weeks on each end of Antares’ “conjunction” with the Sun, which falls around Nov 30th, Antares will be the winner in this category.
It is also worth mentioning that the visibility in “wee” hours (after 1 AM) is being treated equally to the visibility at more friendly hours such as early evening. If we took just the observability at “normal” hours, when each star is not hugging the horizon, the outcome might be totally different.
Milan B, avid sky observer with both SkyWatcher and SkySafari.