2.1 Planning your observations
As you have seen in the previous section, not all of the Messier objects are visible at the same time. Different objects will be visible at different times of the year. Depending on its position in the sky, any given object will be best placed for viewing during certain months of the year and poorly placed or not visible at other times.
To understand this in more detail, you need to think about the Earth’s orbit around the Sun.
Figure 5 shows how the system of right ascension (RA) coordinates relates to the Earth’s position in its orbit at different times of the year. Starting in autumn, a line from the Sun to the Earth on 21 September defines a right ascension of zero. Objects with an RA of zero hours will be highest in the sky at midnight on 21 September. As the Earth moves around the Sun, the angle of the line joining Earth to Sun changes by 30 degrees every month, and this corresponds to a change of two hours of right ascension every month (since there are 12 months in a year and 24 hours in one day).
By the 21 December, objects with an RA of +6 hours are highest at midnight and visible all night. On 21 March, objects with an RA of +12 will be visible all night, and in the summer, on 21 June, objects with an RA of +18 will be visible all night.
Of course, objects with an RA within a few hours either side of these values will also be visible for most of the night and any given object will be visible for a month or two either side of its best position, at least for part of the night.
Activity 3 Predicting visibility
Think about the objects that you looked at in the last exercise:
M31 – The Andromeda galaxy
M42 – The great Orion nebula
M66 – Spiral galaxy in Leo
M11 – The Wild Duck cluster
- In each case, look up the RA and Dec values on the SEDS website that you looked at in Activity 1. Note the RA value and, using Figure 4, decide at what time of year the object would be best placed for imaging with the COAST telescope (remember that COAST is a robotic telescope so can operate for the whole night, from dusk through midnight and on to dawn).
For each object, what is the best time of year to plan to observe using COAST?
As noted in Activity 2, M31 is best placed for imaging with COAST in the autumn, M42 in winter, M66 in the spring and M11 in summer.
- COAST operates all night, but (unless you are a dedicated astronomer) most of us would prefer to observe in the evening rather than at midnight or in the early hours of the morning.
Remembering that any given object rises two hours earlier each month, if an object such as the Orion nebula is at its highest point at midnight in December, in which month would it be best placed for visual observing at 20:00 (8 pm)?
To observe Orion at its highest four hours earlier than midnight the best time would be two months later, so in February.
You can of course check all of this by searching for a particular object in Stellarium, noting its RA coordinate and then adjusting the date and time to find when it is at its highest altitude.