AL Observing Challenges

Picking a winner

Google AI says equatorial mounts were not common at that time. The AI also says Messier’s favorite scopes were:

Primary Telescope: 7.5-inch (190 mm) Gregorian reflector with a focal length of 32 feet (usually magnifying 104x).
Secondary Telescopes: Several 3.5-inch (90 mm) achromatic refractors (usually magnifying 120x).

As a comet hunter, Messier used an Alt/Az mount. Converting Alt/Az coordinates is laborious and error-prone, particularly if not used in transit mode.

The secondary reading supplied by the AL mentions Admiral Smyth’s thought of a possible transcription error in stars Theta Bootes vs. Omicron Bootes. I will ignore that issue. I have not heard of other notation errors by Messier or Méchain.

While looking at the contenders for the true M102, it comes down to looking at the possible contenders for M102 there are three important criteria: location with respect to the mentioned stars, ability to view because of magnitude, and ability to view because of size.

There are certainly better combinations of stars that might have been used, but many of the telescopes at the time of observation were mounted in transit mode because of their large size. That might explain why Iota Draconis and Omicron Boötes were chosen in the description. If M102 were observed on a date and time when it was on or near the meridian, Iota Draconis and Omicron Bootes were close to the meridian too. That would explain why a particular galaxy was noted and others far from the meridian might be eliminated. If true, NGC 3665, M101, and NGC 5928 would be eliminated for consideration. They would be too far from the meridian to be counted. Other star combinations would have been used for them.

There is a link in the PowerPoint I used for outreach that lists 10 telescopes Messier used between 1767 and 1769. Not much would have changed in technology in the years between then and the observation of M102. Many scopes had very long focal lengths (10 + feet), with small apertures and f/#s, and the reflectors had speculum mirrors. That's not a combination that would produce big, bright targets that would jump out of the field. Smaller and dimmer targets would have been very difficult to view. It would be logical to assume the real target would easily be seen in an image taken today with a 17" Slooh telescope. If it were hard to see today, it would have be almost impossible then. Only three of the targets show as an easily seen galaxy in the above images. Those three would be the best bets. That would eliminate NGC 5908, NGC 5907, NGC 5905, and NGC 5979. They are all too small and/or dim to show up in telescopes of that time. Some time ago, I put together a web page showing images of the Messier objects with Slooh scopes. They are generally larger and brighter than most of the above eliminated galaxy images.

The remaining target in the list is NGC 5866. It is the second brightest in the list and the third largest in size. It is my pick for M102. There is no surprise to see that it is usually chosen as M102.

Requirements:

1. Do an observation of the Moon with either Binoculars or a Telescope.   It may be done Eyes Only, but the details will be very small and somewhat difficult to see.

2. The observation must be done between October 2, 2025 and October 11, 2025 to be able to see the four required Maria.

3. The observation may be done visually or through imaging.  Include information on the equipment used.

4. Observe these Maria (dark areas):

   • Mare Serenitatis

   • Mare Tranquilliatatis

   • Mare Imbrium

   • Mare Cognitum

5. Label these four Maria and also the 6 Apollo Mission landing sites.

6. Do an outreach activity to share information about the Apollo Missions or the Moon.  This may also be the Observe the Moon event itself.

7. Submit the required information and the sketch or image to the Coordinator.

8. Deadline for submission is November 11, 2025.  Late entries will not be accepted.

Requirements for this Observing Challenge:

1. Do an Outreach Activity related to the Titan Shadow Transit.

2. Observe a Titan Shadow Transit using a telescope.  Observations may be done any time in 2025, up to the last one on September 20, 2025

3. Sketch or image Titan’s Shadow during the Transit.

4. Identify Titan’s Shadow in the sketch or image.

5. Include information on your telescope and camera (if used).

6. Forward the sketch or image, plus the information on the outreach activity to the AL Observing Challenge Coordinator.  Submission deadline is October 20, 2025.