lemming:
The unit will be solar powered on a hilltop so may experience 0 - 50 degrees Centigrade inside its enclosure depending on the time of day and angle of the sun, so I would have trouble tuning it to a specific temperature.
Let's check the datasheet and see how the processor will do (I assume the processor is actually a 328► P ◄)...
Figure 29-367.ATmega328P: Calibrated 8 MHz RC Oscillator Frequency vs. Temperature
I'll use the 3.0 V curve (the curve shapes are the same and we're only interested in the range not the actual values). Zero is about 7.875 MHz. 50 is about 8.075 MHz. That gives us a range of 0.2 MHz. If you tune for 25 degrees and the tuning gets the processor within 1% of 8 MHz the full range is 7.82 MHz to 8.18 MHz or ±2.25%. A good rule of thumb is ±4% for serial communications. Should work well (no guarantee from the management ;)).
I might resort to a crystal after all, even though they use a bit more power.
If the glove doesn't fit, you must acquit. Oh wait. Wrong catch-phrase. When in doubt, get the crystal out. That's better.
What is the purpose of the clock divisor?
No matter what is providing a clock signal (internal oscillator, crystal, resonator, watchdog oscillator) it can be divided into a lower frequency by changing the clock divisor. In the Arduino world, the divisor is generally only used to reduce the internal oscillator from 8 MHz to 1 MHz. Note: Processors ship from the factory with that configuration (internal divided by 8).
What is the difference between 8 Mhz divided by 8 and using 1 Mhz without divisor?
They are one and the same. The internal oscillator only runs at 8 MHz. By applying a divisor that frequency can be reduced to 1 MHz (eight is not the only divisor; there 9 total divisor values available).