Define the sequence $a_1,a_2,a_3,\ldots$ by $a_n=\sum_{k=1}^n\sin(k)$, where $k$ represents radian measure. Find the index of the $100$th term for which $a_n<0$.
1. Define the sequence \( a_n \) by \( a_n = \sum_{k=1}^n \sin(k) \), where \( k \) is in radians.
2. Multiply the entire sum by \( 2\cos(1) \):
   \[
   2a_n \cos(1) = 2\cos(1) (\sin(1) + \sin(2) + \cdots + \sin(n))
   \]
3. Use the product-to-sum identities to simplify:
   \[
   2\sin(k)\cos(1) = \sin(k+1) + \sin(k-1)
   \]
   Therefore,
   \[
   2a_n \cos(1) = (\sin(2) + \sin(0)) + (\sin(3) + \sin(1)) + \cdots + (\sin(n+1) + \sin(n-1))
   \]
4. Rearrange the terms:
   \[
   2a_n \cos(1) = (\sin(2) + \sin(3) + \cdots + \sin(n)) + (\sin(1) + \sin(2) + \cdots + \sin(n-1)) + \sin(n+1) - \sin(1)
   \]
5. Notice that the sum on the right-hand side is almost \( 2a_n \):
   \[
   2a_n \cos(1) = 2a_n - \sin(n) - \sin(1) + \sin(n+1)
   \]
6. Solve for \( a_n \):
   \[
   2a_n \cos(1) = 2a_n - \sin(n) - \sin(1) + \sin(n+1)
   \]
   \[
   2a_n \cos(1) - 2a_n = -\sin(n) - \sin(1) + \sin(n+1)
   \]
   \[
   a_n (2\cos(1) - 2) = \sin(n+1) - \sin(n) - \sin(1)
   \]
   \[
   a_n = \frac{\sin(n+1) - \sin(n) - \sin(1)}{2(\cos(1) - 1)}
   \]
7. Determine when \( a_n < 0 \):
   \[
   \sin(n+1) - \sin(n) - \sin(1) < 0
   \]
   \[
   \sin(n+1) < \sin(n) + \sin(1)
   \]
8. Use the trigonometric identity for the sine function:
   \[
   \sin(n+1) = \sin(n)\cos(1) + \cos(n)\sin(1)
   \]
   \[
   \sin(n)\cos(1) + \cos(n)\sin(1) < \sin(n) + \sin(1)
   \]
   \[
   \sin(n)(\cos(1) - 1) < \sin(1) - \cos(n)\sin(1)
   \]
   \[
   \sin(n)(\cos(1) - 1) < \sin(1)(1 - \cos(n))
   \]
   \[
   \sin(n) < \frac{\sin(1)(1 - \cos(n))}{\cos(1) - 1}
   \]
9. Simplify the inequality:
   \[
   \cos\left(n + \frac{1}{2}\right) > \cos\left(\frac{1}{2}\right)
   \]
10. Reduce \( n \) modulo \( 2\pi \):
    \[
    n \in (2k\pi - 1, 2k\pi)
    \]
    for any positive integer \( k \).
11. Find the index of the 100th term for which \( a_n < 0 \):
    \[
    n \in (200\pi - 1, 200\pi)
    \]
    Since \( 200\pi \approx 628.318 \), the integer in this range is \( 628 \).

The final answer is \( \boxed{628} \)