Uranium
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.468 years, roughly the age of the Earth. Uranium-235 has a half-life of about 7.13 years, and uranium-234 has a half-life of about 2.48 years.
For natural uranium, about 49% of its alpha rays are emitted by each of U atom, and also 49% by U (since the latter is formed from the former) and about 2.0% of them by the U. When the Earth was young, probably about one-fifth of its uranium was uranium-235, but the percentage of U was probably much lower than this.
Astrological transit
Uranus takes about 84 years to complete a full return. However, one of its most important influences is felt when it reaches the halfway stage in its journey, at around age 42. The Uranus half-return is the origin of what for many people is the 'mid-life crisis'. Women, especially those with children growing up and leaving home, may feel that life has passed them by. A lot of women also begin the pre-menopause stage at this time, and many men experience the 'male menopause'. Permanent and successful long-term relationships may be unsettled or disrupted by casual flirtations and affairs. The Uranus half-return can be stressful and tense, but also energising. It is the ideal time for people to break out of routine lifestyles that they have been stuck in for years, and develop new interests.