Donati's Horoscope

Donati's Horoscope (ドナーティの 天体観測図 (ホロスコープ) ) is a horoscope that was retrieved by Academy City as a scientific item, but was also a key to acquire the knowledge of the Fragment of the Rune Epitaph.

Principles
Donati's Horoscope had been the sole existing horoscope that had been discovered and retrieved by Academy City as a purely scientific item, making it a slight issue between the science side and the magic side. The Horoscope in question was a key item for the complete recovery of the Alaskan rune epitaph and the acquisition of its knowledge.

The horoscope can be used to calculate the magical effects of Donati's Comet.

Toaru Majutsu no Index SS: Stiyl
To acquire the knowledge of the Alaskan runes, Richard Brave needed the Fragment of the Rune Epitaph that Stiyl had and the horoscope that could be used to calculate the magical effects of Donati’s Comet which only approached the earth after a period of longer than 2000 years. The lack of any one of those things was fatal to the his plan however, the epitaph fragment was a portion of an Original Grimoire, so it could not be destroyed by human means and using Patricia’s life as a shield was not an option for him.

Stiyl had no choice but to steal the horoscope that was being transported by Academy City, if he managed that before the Richard could do so, he could come up with a plan using it as bait. Unfortunately for both men it had already been stolen by Leivinia Birdway and Dawn-Colored Sunlight.

Later Stiyl confronted Birdway as to what they would do with the Epitaph and the Horoscope, she responded in saying that they had no interest in Norse mythology at all, and the techniques of the Dvergr would be of no use to them even if they had them. The Horoscope originally belonged to Dawn-Colored Sunlight and they had purposefully allowed Academy City to find it and recover it. They have all the three requirements to carry out the ceremony from the beginning (someone from Birdway's bloodline, the epitaph fragment, and the horoscope), and if they were planning to carry it out, they would have already done so and thought that sending the horoscope to the headquarters of the science side would be best because it would get the best care in a world that had no idea what its magical value was. Though Academy City turned out to be a surprisingly bad choice and they determined it couldn't be left in the city thus they came to retrieve it.